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  • 151. Roca-Sanjuan, Daniel
    et al.
    Farahani, Pooria
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Frances-Monerris, Antonio
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.
    Engineering Chemiluminescent Properties by Means of Non-Adiabatic Computational Chemistry2014In: Luminescence (Chichester, England Print), ISSN 1522-7235, E-ISSN 1522-7243, Vol. 29, p. 43-43Article in journal (Other academic)
  • 152.
    Roca-Sanjuan, Daniel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lundberg, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Mazziotti, D. A.
    Univ Chicago, James Franck Inst, Dept Chem, Chicago, IL 60637 USA.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Comment on "€œDensity functional theory study of 1,2-€dioxetanone decomposition in condensed phase€"2012In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 33, no 26, p. 2124-2126Article in journal (Refereed)
    Abstract [en]

    In the preceding paper results are presented, which are in serious conflict with state-of-the-art ab initio method. Based on these new results the authors propose a new explanation of the reason for the preferential production of a phosphorescent state. Here we show that these controversial results are flawed, since the model use exclude biradical electron structures.

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  • 153.
    Roca-Sanjuán, Daniel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theortical Chemistry.
    Delcey, Mickaël G.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theortical Chemistry.
    Navizet, Isabelle
    Ferre, Nicolas
    Liu, Ya-Jun
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theortical Chemistry.
    Chemiluminescence and Fluorescence States of a Small Model for Coelenteramide and Cypridina Oxyluciferin: A CASSCF/CASPT2 Study2011In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 7, no 12, p. 4060-4069Article in journal (Refereed)
    Abstract [en]

    Fluorescence and chemiluminescence phenomena are often confused in experimental and theoretical studies on the luminescent properties of chemical systems. To establish the patterns that distinguish both processes, the fluorescent and chemiluminescent states of 2-acetamido-3-methylpyrazine, which is a small model of the coelenterazine/coelenteramide and Cypridina luciferin/oxyluciferin bioluminescent systems, were characterized by using the complete active space second-order perturbation (CASPT2) method. Differences in geometries and electronic structures among the states responsible for light emission were found. On the basis of the findings, some recommendations for experimental studies on chemiluminescence are suggested, and more appropriate theoretical approaches are proposed.

  • 154. Roca-Sanjuán, Daniel
    et al.
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Giussani, Angelo
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    A Theoretical Analysis of the Intrinsic Light-Harvesting Properties of Xanthopterin2014In: Computational and Theoretical Chemistry, ISSN 2210-271X, E-ISSN 2210-2728, Vol. 1040-1041, p. 230-236Article in journal (Refereed)
    Abstract [en]

    Belonging to the family of pterins, which are common chromophores in several bio-organisms, xanthopterin has been shown experimentally (Plotkin et al., 2010) to have the ability of acting as a light-harvesting molecule. In the present study, multiconfigurational second-order perturbation theory is used to determine the stability of distinct amino/imino and lactam/lactim tautomers and the absorption and emission spectroscopic characteristics, electron donor and acceptor properties and the electron and charge transfer efficiencies via π-stacking. The lactam–lactam form 3H5H (and in a lesser extent 1H5H) is predicted to have the most appropriate intrinsic characteristics for the light-harvesting properties of xanthopterin, since it is the most stable isomer predicted for the gas phase and estimated for polar environments, absorbs solar light at longer wave lengths, has relatively low donor properties and the presence of the keto groups, instead of enol, increases the efficiency for energy transfer through excimer-like interactions.

  • 155. Roca-Sanjuán, Daniel
    et al.
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Liu, Ya-Jun
    Recent method developments and applications in computational photochemistry, chemiluminescence and bioluminescence2015In: Photochemistry: Volume 42 / [ed] Elisa Fasani, Angelo Albini, Royal Society of Chemistry, 2015, p. 11-42Chapter in book (Other academic)
    Abstract [en]

    This review summarises and discusses the advances of computational photochemistry in 2012 and 2013 in both methodology and applications fields. The methodological developments of models and tools used to study and simulate non-adiabatic processes are highlighted. These developments can be summarised as assessment studies, new methods to locate conical intersections, tools for representation, interpretation and visualisation, new computational approaches and studies introducing simpler models to rationalise the quantum dynamics near and in the conical intersection. The applied works on the topics of photodissociation, photostability, photoisomerisations, proton/charge transfer, chemiluminescence and bioluminescence are summarised, and some illustrative examples of studies are analysed in more detail, particularly with reference to photostability and chemi/bioluminescence. In addition, theoretical studies analysing solvent effects are also considered. We finish this review with conclusions and an outlook on the future.

  • 156. Roca-Sanjuán, Daniel
    et al.
    Francés-Monerris, Antonio
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Farahani, Pooria
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Liu, Ya-Jun
    Advances in computational photochemistry and chemiluminescence of biological and nanotechnological molecules2017In: Photochemistry: Volume 44 / [ed] Angelo Albini, Elisa Fasani, Cambridge, UK: Royal Society of Chemistry, 2017, p. 16-60Chapter in book (Other academic)
    Abstract [en]

    Recent advances (2014–2015) in computational photochemistry and chemiluminescence derive from the development of theory and from the application of state-of-the-art and new methodology to challenging electronic-structure problems. Method developments have mainly focused, first, on the improvement of approximate and cheap methods to provide a better description of non-adiabatic processes, second, on the modification of accurate methods in order to decrease the computation time and, finally, on dynamics approaches able to provide information that can be directly compared with experimental data, such as yields and lifetimes. Applications of the ab initio quantum-chemistry methods have given rise to relevant findings in distinct fields of the excited-state chemistry. We briefly summarise, in this chapter, the achievements on photochemical mechanisms and chemically-induced excited-state phenomena of interest in biology and nanotechnology.

  • 157. Roos, Björn O
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Cho, Han-Gook
    Andrews, Lester
    Agostic interaction in the methylidene metal dihydride complexes H2MCH2 (M = Y, Zr, Nb, Mo, Ru, Th, or U)2007In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 111, no 28, p. 6420-6424Article in journal (Refereed)
    Abstract [en]

    Multiconfigurational quantum chemical methods (complete active space self-consistent field (CASSCF)/second-order perturbation theory (CASPT2)) have been used to study the agostic interaction between the metal atom and H(C) in the methylidene metal dihydride complexes H2MCH2, where M is a second row transition metal or the actinide atoms Th or U. The geometry of some of these complexes is highly irregular due to the formation of a three center bond CH center dot center dot center dot M, where the electrons in the CH bond are delocalized onto empty or half empty orbitals of d- or f-type on the metal. No agostic interaction is expected when M = Y, where only a single bond with methylene can be formed, or when M = Ru, because of the lack of empty electron accepting metal valence orbitals. The largest agostic interaction is found in the Zr and U complexes.

  • 158. Roos, Björn O
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Malmqvist, Per-Åke
    Veryazov, Valera
    Widmark, Per-Olof
    Main group atoms and dimers studied with a new relativistic ANO basis set2004In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 108, no 15, p. 2851-2858Article in journal (Refereed)
    Abstract [en]

    New basis sets of the atomic natural orbital (ANO) type have been developed for the main group and rare gas atoms. The ANO’s have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the dimer at its equilibrium geometry. Scalar relativistic effects are included through the use of a Douglas-Kroll Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and the ground-state potentials for the dimers. These calculations include spin-orbit coupling using the RASSCF State Interaction (RASSI-SO) method. The spin-orbit splitting for the lowest atomic term is reproduced with an accuracy of better than 0.05 eV, except for row 5, where it is 0.15 eV. Ionization energies and electron affinities have an accuracy better than 0.2 eV, and atomic polarizabilities for the spherical atoms are computed with errors smaller than 2.5%. Computed bond energies for the dimers are accurate to better than 0.15 eV in most cases (the dimers for row 5 excluded).

  • 159. Roos, Björn O
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Malmqvist, Per-Åke
    Veryazov, Valera
    Widmark, Per-Olof
    New relativistic ANO basis sets for actinide atoms2005In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 409, no 4-6, p. 295-299Article in journal (Refereed)
    Abstract [en]

    New basis sets of the atomic natural orbital (ANO) type have been developed for the actinide atoms Ac-Cm. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in a electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The lowest multiplet levels have been computed. These calculations include spin-orbit coupling using a variation-perturbation approach. The atomic polarizability of the spherically symmetric americium atom has been computed to be 116 au(3).

  • 160. Roos, Björn O
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Malmqvist, Per-Åke
    Veryazov, Valera
    Widmark, Per-Olof
    New relativistic ANO basis sets for transition metal atoms2005In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 109, no 29, p. 6575-6579Article in journal (Refereed)
    Abstract [en]

    New basis sets of the atomic natural orbital (ANO) type have been developed for the first, second, and third row transition metal atoms. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive and negative ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies, electron affinities, and excitation energies for all atoms and polarizabilities for spherically symmetric atoms. These calculations include spin-orbit coupling using a variation-perturbation approach. Computed ionization energies have an accuracy better than 0.2 eV in most cases. The accuracy of computed electron affinities is the same except in cases where the experimental values are smaller than 0.5 eV. Accurate results are obtained for the polarizabilities of atoms with spherical symmetry. Multiplet levels are presented for some of the third row transition metals.

  • 161. Roos, Björn O
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Malmqvist, Per-Åke
    Veryazov, Valera
    Widmark, Per-Olof
    Borin, Antonio Carlos
    New Relativistic Atomic Natural Orbital Basis Sets for Lanthanide Atoms with Applications to the Ce Diatom and LuF32008In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, no 45, p. 11431-11435Article in journal (Refereed)
    Abstract [en]

    New basis sets of the atomic natural orbital (ANO) type have been developed for the lanthanide atoms La-Lu. The ANOs have been obtained from the average density matrix of the ground and lowest excited states of the atom, the positive ions, and the atom in an electric field. Scalar relativistic effects are included through the use of a Douglas-Kroll-Hess Hamiltonian. Multiconfigurational wave functions have been used with dynamic correlation included using second-order perturbation theory (CASSCF/CASPT2). The basis sets are applied in calculations of ionization energies and some excitation energies. Computed ionization energies have an accuracy better than 0.1 eV in most cases. Two molecular applications are inluded as illustration: the cerium diatom and the LuF3 molecule. In both cases it is shown that 4f orbitals are not involved in the chemical bond in contrast to an earlier claim for the latter molecule.

  • 162. Rubio, Mercedes
    et al.
    Stålring, Jonna
    Bernhardsson, Anders
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Roos, Björn O
    Theoretical studies of isomers of C3H2 using a multiconfigurational approach2000In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 105, no 1, p. 15-30Article in journal (Refereed)
    Abstract [en]

    The C3H2 isomers are important molecules in interstellar space. An understanding of their electronic structure can contribute significantly to the interpretation of interstellar spectra. In a theoretical study of the C3H2 isomers a multiconfigurational treatment is of interest because many of the isomers are carbenes or diradicals. We present such an investigation of all possible C3H2 isomers. The most important features of their electronic and vibrational spectra are calculated. Earlier theoretical studies are reviewed and it is shown that the present study yields the same order of stability for the singlet and triplet states as most previous studies.

  • 163. Ryu, U
    et al.
    Lee, Y S
    Lindh, Roland
    Department of Theoretical Chemistry, University of Lund Chemical Center.
    An efficient method of implementing the horizontal recurrence relation in the evaluation of electron repulsion integrals using Cartesian Gaussian functios1991In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 185, no 5-6, p. 562-568Article in journal (Refereed)
    Abstract [en]

    The tree search problem is considered for optimizing the horizontal recurrence relation step, which is a three-dimensional recurrence relation used in generating two-electron integrals. By eliminating redundant work, it is possible to achieve 13%, 25%, 38% and 44% savings in floating point operations and 21%, 34%, 46% and 53% reductions in memory requirements for the (ff\textbackslashff), (gg\textbackslashgg), (hh\textbackslashhh) and (ii\textbackslashii) cases, respectively. These savings and reductions will lead to quite efficient two-electron integral and derivative programs when high angular momentum functions are included.

  • 164.
    Sand, Andrew M.
    et al.
    Univ Minnesota, Dept Chem, Chem Theory Ctr, 207 Pleasant St SE, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Hoyer, Chad E.
    Univ Minnesota, Dept Chem, Chem Theory Ctr, 207 Pleasant St SE, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Sharkas, Kamal
    Univ Minnesota, Dept Chem, Chem Theory Ctr, 207 Pleasant St SE, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Kidder, Katherine M.
    Univ Minnesota, Dept Chem, Chem Theory Ctr, 207 Pleasant St SE, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Truhlar, Donald G.
    Univ Minnesota, Dept Chem, Chem Theory Ctr, 207 Pleasant St SE, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Gagliard, Laura
    Univ Minnesota, Dept Chem, Chem Theory Ctr, 207 Pleasant St SE, Minneapolis, MN 55455 USA.;Univ Minnesota, Minnesota Supercomp Inst, Minneapolis, MN 55455 USA..
    Analytic Gradients for Complete Active Space Pair-Density Functional Theory2018In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 14, no 1, p. 126-138Article in journal (Refereed)
    Abstract [en]

    Analytic gradient routines are a desirable feature for quantum mechanical methods, allowing for efficient determination of equilibrium and transition state structures and several other molecular properties. In this work, we present analytical gradients for multiconfiguration pair-density functional theory (MC-PDFT) when used with a state-specific complete active space self-consistent field reference wave function. Our approach constructs a Lagrangian that is variational in all wave function parameters. We find that MC-PDFT locates equilibrium geometries for several small- to medium-sized organic molecules that are similar to those located by complete active space second-order perturbation theory but that are obtained with decreased computational cost.

  • 165. Sankari, Anna
    et al.
    Stråhlman, Christian
    Sankari, Rami
    Partanen, Leena
    Laksman, Joakim
    Kettunen, J. Antti
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Organic Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Organic Chemistry.
    Malmqvist, Per-Åke
    Sorensen, Stacey L.
    Non-radiative decay and fragmentation in water molecules after 1a1-14a1 excitation and core ionization studied by electron-energy-resolved electron–ion coincidence spectroscopy2020In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 152, no 7, article id 074302Article in journal (Refereed)
    Abstract [en]

    In this paper, we examine decay and fragmentation of core-excited and core-ionized water molecules combining quantum chemical calculations and electron-energy-resolved electron–ion coincidence spectroscopy. The experimental technique allows us to connect electronic decay from core-excited states, electronic transitions between ionic states, and dissociation of the molecular ion. To this end, we calculate the minimum energy dissociation path of the core-excited molecule and the potential energy surfaces of the molecular ion. Our measurements highlight the role of ultra-fast nuclear motion in the 1a1-14a1 core-excited molecule in the production of fragment ions. OH+ fragments dominate for spectator Auger decay. Complete atomization after sequential fragmentation is also evident through detection of slow H+ fragments. Additional measurements of the non-resonant Auger decay of the core-ionized molecule (1a1-1) to the lower-energy dication states show that the formation of the OH+ + H+ ion pair dominates, whereas sequential fragmentation OH+ + H+ → O + H+ + H+ is observed for transitions to higher dication states, supporting previous theoretical investigations.

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  • 166.
    Sauri, Vicenta
    et al.
    Univ Valencia, Inst Ciencia Mol, ES-46071 Valencia, Spain.
    Gobbo, João P.
    Univ Sao Paulo, Inst Quim, BR-05508900 Sao Paulo, Brazil;USP Consortium Photochem Technol, NAPPhotoTech, BR-05508900 Sao Paulo, Brazil.
    Serrano-Pérez, Juan J.
    Univ London Imperial Coll Sci Technol & Med, Dept Chem, London SW7 2AZ, England.
    Lundberg, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Coto, Pedro B.
    Univ Valencia, Inst Ciencia Mol, ES-46071 Valencia, Spain; Univ Erlangen Nurnberg, Inst Theoret Phys, D-91058 Erlangen, Germany; Univ Alcala de Henares, Dept Quim Fis, E-28871 Alcala De Henares, Spain.
    Serrano-Andrés, Luis
    Univ Valencia, Inst Ciencia Mol, ES-46071 Valencia, Spain.
    Borin, Antonio C.
    Univ Sao Paulo, Inst Quim, BR-05508900 Sao Paulo, Brazil;USP Consortium Photochem Technol, NAPPhotoTech, BR-05508900 Sao Paulo, Brazil.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Merchán, Manuela
    Univ Valencia, Inst Ciencia Mol, ES-46071 Valencia, Spain.
    Roca-Sanjuán, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Proton/Hydrogen Transfer Mechanisms in the Guanine-€“Cytosine Base Pair: Photostability and Tautomerism2013In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 9, no 1, p. 481-496Article in journal (Refereed)
    Abstract [en]

    Proton/hydrogen-transfer processes have been broadly studied in the past 50 years to explain the photostability and the spontaneous tautomerism in the DNA base pairs. In the present study, the CASSCF/CASPT2 methodology is used to map the two-dimensional potential energy surfaces along the stretched NH reaction coordinates of the guanine–cytosine (GC) base pair. Concerted and stepwise pathways are explored initially in vacuo, and three mechanisms are studied: the stepwise double proton transfer, the stepwise double hydrogen transfer, and the concerted double proton transfer. The results are consistent with previous findings related to the photostability of the GC base pair, and a new contribution to tautomerism is provided. The C-based imino-oxo and imino-enol GC tautomers, which can be generated during the UV irradiation of the Watson–Crick base pair, have analogous radiationless energy-decay channels to those of the canonical base pair. In addition, the C-based imino-enol GC tautomer is thermally less stable. A study of the GC base pair is carried out subsequently taking into account the DNA surroundings in the biological environment. The most important stationary points are computed using the quantum mechanics/molecular mechanics (QM/MM) approach, suggesting a similar scenario for the proton/hydrogen-transfer phenomena in vacuo and in DNA. Finally, the static model is complemented by ab initio dynamic simulations, which show that vibrations at the hydrogen bonds can indeed originate hydrogen-transfer processes in the GC base pair. The relevance of the present findings for the rationalization of the preservation of the genetic code and mutagenesis is discussed.

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  • 167.
    Schalk, O.
    et al.
    Stockholm Univ, AlbaNova Univ Ctr, Dept Chem Phys, S-10691 Stockholm, Sweden..
    Stenrup, Michael
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry.
    Geng, T.
    Stockholm Univ, AlbaNova Univ Ctr, Dept Chem Phys, S-10691 Stockholm, Sweden..
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Thomas, R. D.
    Stockholm Univ, AlbaNova Univ Ctr, Dept Chem Phys, S-10691 Stockholm, Sweden..
    Feifel, R.
    Univ Gothenburg, Dept Phys, S-41296 Gothenburg, Sweden..
    Hansson, T.
    Stockholm Univ, AlbaNova Univ Ctr, Dept Chem Phys, S-10691 Stockholm, Sweden..
    Influence of Alkoxy Groups on the Photoinduced Dynamics of Organic Molecules Exemplified on Alkyl Vinyl Ethers2015In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 119, no 45, p. 11105-11112Article in journal (Refereed)
    Abstract [en]

    A series of different alkyl vinyl ethers is investigated to decipher the possible reaction channels upon photoexcitation to the pi 3s-Rydberg and the pi pi*-valence state at 200 nm using time-resolved photoelectron spectroscopy and on-the-fly time-dependent density functional theory dynamics simulations. The results indicate two possible relaxation pathways: (1) a radiationless decay through the pi pi*-state back to the ground state via torsion of the C=C double bond, in accordance with the dynamics found in ethylene; and (2) a fast dissociation of the C-O bond between the alkyl and the vinoxy group in the pi sigma*-state. The latter state can be accessed only after excitation to the pi 3s-Rydberg state (quantum yield of similar to 50% according to the dynamics simulations). Additionally, the excited state barrier leading to formation of a vinyl radical was found to be too high to be crossed. These results indicate that the dynamics of ethers crucially depend on the excitation wavelength and that the pi sigma*-state constitutes an important competitive reaction channel that leads to dissociation of the molecules.

  • 168. Schapiro, Igor
    et al.
    Roca-Sanjuán, Daniel
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Olivucci, Massimo
    A surface hopping algorithm for nonadiabatic minimum energy path calculations2015In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 36, no 5, p. 312-20Article in journal (Refereed)
    Abstract [en]

    The article introduces a robust algorithm for the computation of minimum energy paths transiting along regions of near-to or degeneracy of adiabatic states. The method facilitates studies of excited state reactivity involving weakly avoided crossings and conical intersections. Based on the analysis of the change in the multiconfigurational wave function the algorithm takes the decision whether the optimization should continue following the same electronic state or switch to a different state. This algorithm helps to overcome convergence difficulties near degeneracies. The implementation in the MOLCAS quantum chemistry package is discussed. To demonstrate the utility of the proposed procedure four examples of application are provided: thymine, asulam, 1,2-dioxetane, and a three-double-bond model of the 11-cis-retinal protonated Schiff base.

  • 169. Schapiro, Igor
    et al.
    Ryanzantsev, Mikhail Nikolaevich
    Frutos, Luis Manuel
    Ferré, Nicolas
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Quantum Chemistry.
    Olivucci, Massimo
    The Ultrafast Photoisomerizations of Rhodopsin and Bathorhodopsin Are Modulated by Bond Length Alternation and HOOP Driven Electronic Effects2011In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 133, no 10, p. 3354-3364Article in journal (Refereed)
    Abstract [en]

    Rhodopsin (Rh) and bathorhodopsin (bathoRh) quantum-mechanics/molecular-mechanics models based on ab initio multiconfigurational wave functions are employed to look at the light induced π-bond breaking and reconstitution occurring during the RhfbathoRh and bathoRhfRh isomerizations. More specifically, semiclassical trajectory computations are used to compare the excited (S1) and ground (S0) state dynamics characterizing the opposite steps of the Rh/bathoRh photochromic cycle during the first 200 fs following photoexcitation. We show that the information contained in these data provide an unprecedented insight into the sub-picosecond π-bond reconstitution process which is at the basis of the reactivity of the protein embedded 11-cis and all-trans retinal chromophores.More specifically, the data point to the phase and amplitude of the skeletal bond length alternation stretchingmode as the key factor switching the chromophore to a bonding state. It is also confirmed/found that the phase and amplitude of the hydrogen-out-of-plane mode controls the stereochemical outcome of the forward and reverse photoisomerizations.

  • 170. Schapiro, Igor
    et al.
    Ryazantsev, Mikhail Nikolaevich
    Manuel Frutos, Luis
    Ferre, Nicolas
    Lindh, Roland
    Olivucci, Massimo
    Using trajectories to probe the molecular mechanism of ultrafast biological photoisomerization and internal conversion2011In: Abstracts of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 242Article in journal (Refereed)
  • 171. Schütz, Martin
    et al.
    Brdarski, Steve
    Widmark, Per-Olof
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Karlström, Gunnar
    The water dimer interaction energy: Convergence to the basis set limit at the correlated level1997In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 107, no 12, p. 4597-4605Article in journal (Refereed)
    Abstract [en]

    The water dimer interaction energy and its convergence to the basis set limit was investigated, with electron correlation effects treated at the level of second order Moller-Plesset perturbation theory (MP2). ANO-type and large uncontracted basis sets were used, spreading over a wide range in size; the biggest set included 1046 functions with angular momentum up to (l=7). Core correlation effects were treated accurately by augmenting the original valence basis with extended sets of core polarization functions. The MP2 dimer interaction energy at the basis set limit was determined to -4.94+/-0.02 kcal/mol, with a contribution due to core correlation of -0.04 kcal/mol. Furthermore, based on some elementary considerations from intermolecular perturbation theory, a simple procedure was devised, which brings the counterpoise corrected interaction energies of moderate basis set calculations closer to the basis set limit. The interaction energies so obtained turned out be surprisingly stable with respect to extensions of the basis set.

  • 172. Schütz, Martin
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    An integral direct, distributed-data, parallel MP2 algorithm1997In: THEORETICA CHIMICA ACTA, ISSN 0040-5744, Vol. 95, no 1-2, p. 13-34Article in journal (Refereed)
    Abstract [en]

    A scalable integral direct, distributed-data parallel algorithm for four-index transformation is presented. The algorithm was implemented in the context of the second-order Moller-Plesset (MP2) energy evaluation, yet it is easily adopted for other electron correlation methods, where only MO integrals with two indices in the virtual orbitals space are required. The major computational steps of the MP2 energy are the two-electron integral evaluation O(N-4) and transformation into the MO basis O(ON4) where N is the number of basis functions, and O the number of occupied orbitals, respectively. The associated maximal communication costs scale as O(n(Sigma)O(2)V N), where V and n(Sigma) denote the number of virtual orbitals, and the number of symmetry-unique shells. The largest local and global memory requirements are Co(N-2) for the MO coefficients and O(OV N) for the three-quarter transformed integrals, respectively. Several aspects of the implementation such as symmetry-treatment, integral prescreening, and the distribution of data and computational tasks are discussed. The parallel efficiency of the algorithm is demonstrated by calculations on the phenanthrene molecule, with 762 primitive Gaussians, contracted to 412 basis functions. The calculations were performed on an IBM SP2 with 48 nodes. The measured wall clock time on 48 nodes is less than 15 min for this calculation, and the speedup relative to single-node execution is estimated to 527. This superlinear speedup is a result of exploiting both the compute power and the aggregate memory of the parallel computer. The tatter reduces the number of passes through the AO integral list, and hence the operation count of the calculation. The test calculations also show that the evaluation of the two-electron integrals dominates the calculation, despite the higher scaling of the transformation step.

  • 173. Schütz, Martin
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Werner, Hans-Joachim
    Integral-direct electron correlation methods1999In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 96, no 4, p. 719-733Article in journal (Refereed)
    Abstract [en]

    Integral-direct implementations of Moller-Plesset perturbation theory up to fourth order (MP4(SDQ) without triple excitations), quadratic configuration interaction with single and double excitations (QCISD), coupled cluster (CCSD), multiconfiguration self-consistent field (MCSCF), second- and third-order multireference perturbation theory (MRPT2, MRPT3), and internally contracted multireference configuration interaction (MRCI) methods are described. All methods use the same three integral-direct kernels, namely direct calculation of Fock matrices, direct partial integral transformation, and direct evaluation of external exchange matrices. New algorithms for the latter two procedures are proposed and implemented, for which the memory requirement scales only linearly with basis set size. Test calculations with more than 500 basis functions are presented. It is demonstrated that integral prescreening can significantly reduce the scaling of the computational cost with molecular size N. For small polyglycine peptides (Gly)(n) (n = 1-4), we observe a scaling of N-3 for the direct integral transformation (formal scaling N-5), and N-4 for the direct computation of the external exchange matrices (formal scaling N-6).

  • 174. Schütz, Martin
    et al.
    Werner, Hans-Joachim
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Manby, Frederick R
    Analytical energy gradients for local second-order Moller-Plesset perturbation theory using density fitting approximations2004In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 121, no 2, p. 737-750Article in journal (Refereed)
    Abstract [en]

    An efficient method to compute analytical energy derivatives for local second-order Moller-Plesset perturbation energy is presented. Density fitting approximations are employed for all 4-index integrals and their derivatives. Using local fitting approximations, quadratic scaling with molecular size and cubic scaling with basis set size for a given molecule is achieved. The density fitting approximations have a negligible effect on the accuracy of optimized equilibrium structures or computed energy differences. The method can be applied to much larger molecules and basis sets than any previous second-order Moller-Plesset gradient program. The efficiency and accuracy of the method is demonstrated for a number of organic molecules as well as for molecular clusters. Examples of geometry optimizations for molecules with 100 atoms and over 2000 basis functions without symmetry are presented. (C) 2004 American Institute of Physics.

  • 175. Segarra-Marti, Javier
    et al.
    Roca-Sanjuan, Daniel
    Merchan, Manuela
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    On the photophysics and photochemistry of the water dimer2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 24, p. 244309-Article in journal (Refereed)
    Abstract [en]

    The photochemistry of the water dimer irradiated by UV light is studied by means of the complete active space perturbation theory//complete active space self-consistent field (CASPT2//CASSCF) method and accurate computational approaches like as minimum energy paths. Both electronic structure computations and ab initio molecular dynamics simulations are carried out. The results obtained show small shifts relative to a single water molecule on the vertical excitation energies of the dimer due to the hydrogen bond placed between the water donor (W-D) and the water acceptor (W-A). A red-shift and a blue-shift are predicted for the W-D and W-A, respectively, supporting previous theoretical and experimental results. The photoinduced chemistry of the water dimer is described as a process occurring between two single water molecules in which the effect of the hydrogen bond plays a minor role. Thus, the photoinduced decay routes correspond to two photodissociation processes, one for each water molecule. The proposed mechanism for the decay channels of the lowest-lying excited states of the system is established as the photochemical production of a hydrogen-bonded H2O center dot center dot center dot HO species plus a hydrogen H atom.

  • 176. Serrando-Andrés, Luis
    et al.
    Merchán, Manuela
    Roos, Björn O
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Theoretical Study of the Internal Charge Transfer in Aminobenzonitriles1995In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 117, no 11, p. 3189-3204Article, review/survey (Refereed)
    Abstract [en]

    The lower excited states for the molecules aminobenzonitrile (ABN) and (dimethylamino)benzonitrile (DMABN) have been studied as a function of the twisting and wagging motion of the amino group. Theoretical calculations have been performed using the complete active space (GAS) SCF method in combination with multiconfigurational second Order perturbation theory (CASPT2): Basis sets of the ANO-type (C,N/3s2p1d and H/2s) were employed. Ground state geometries were optimized at the CASSCF level. The excitation energies were computed as function of a twist angle, where the amino group is rotated with respect to the benzonitrile plane, and for two values of the wagging angle (0 and 21 degrees). The influence of the wagging angle in the nontwisted molecules was also analyzed. The results fully confirm the twist intramolecular charge transfer (TICT) model proposed to explain the dual fluorescence phenomena occurring in DMABN. The absence of the low frequency part of the fluorescence spectrum in ABN in explained by the shape of the potential energy surface along the isomerization path, due to the large energy gaps among the interacting states, which prevents the amino group from rotating into the TICT state. Calculated transition energies (absorption and emission), structural, and electrical properties of the ground and excited states are in agreement with available experimental information.

  • 177. Serrano-Andres, Luis
    et al.
    Merchán, Manuela
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Computation of conical intersections by using perturbation techniques2005In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 122, no 10, p. 104107-Article in journal (Refereed)
    Abstract [en]

    Multiconfigurational second-order perturbation theory, both in its single-state multiconfigurational second-order perturbation theory (CASPT2) and multistate (MS-CASPT2) formulations, is used to search for minima on the crossing seams between different potential energy hypersurfaces of electronic states in several molecular systems. The performance of the procedures is tested and discussed, focusing on the problem of the nonorthogonality of the single-state perturbative solutions. In different cases the obtained structures and energy differences are compared with available complete active space self-consistent field and multireference configuration interaction solutions. Calculations on different state crossings in LiF, formaldehyde, the ethene dimer, and the penta-2,4-dieniminium cation illustrate the discussions. Practical procedures to validate the CASPT2 solutions in polyatomic systems are explored, while it is shown that the application of the MS-CASPT2 procedure is not straightforward and requires a careful analysis of the stability of the results with the quality of the reference wave functions, that is, the size of the active space.

  • 178. Serrano-Andrés, Luis
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Roos, Björn O
    Merchan, Manuela
    Theoretical study of the electronic spectrum of all-trans-1,3,5,7-octatetraene1993In: The journal of physical chemistry, ISSN 0022-3654, Vol. 97, no 37, p. 9360-9368Article in journal (Refereed)
    Abstract [en]

    Ab initio calculations have been performed for the electronic spectrum of all-trans-1,3,5,7-octatetraene including geometry optimizations for the ground and low-lying valence excited singlet states at the complete active space self-consistent-field level. Excitation energies have been calculated using multiconfigurational second-order perturbation theory (CASPT2). Computed excitation energies for the valence and a number of low-lying Rydberg excited singlet and triplet states make possible confident assignments of the main features reported in the experimental spectra. The computed vertical and nonvertical excitations and the fluorescence maxima confirm the suggested presence of the 2(1)A(g) valence state below the 1 1B(u) optically allowed state, as is the characteristic for the longer polyenes. The agreement is within 0.05 eV for all excitation energies where comparison with experimental data is possible.

  • 179. Serrano-Andrés, Luis
    et al.
    Merchán, Manuela
    Nebot-Gil, Ignacio
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Roos, Björn O
    Towards an accurate molecular orbital theory for excited states: Ethene, butadiene, and hexatriene1993In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 98, no 4, p. 3151-3162Article in journal (Refereed)
    Abstract [en]

    A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans-1,3-butadiene, and trans-trans-1,3,5-hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 B-1(u) state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 eV.

  • 180. Siegbahn, Per
    et al.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Quantum Chemistry.
    Bjorn O. Roos: 1937-2010 Mentor, Colleague, Innovator2011In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 111, no 13, p. 3256-3259Article in journal (Refereed)
  • 181.
    Stegeby, Henrik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Piszczatowski, Konrad
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Karlsson, Hans O.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Froelich, Piotr
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Variational calculations for the hydrogen-antihydrogen system with a mass-scaled Born-Oppenheimer potential2012In: CENTRAL EUROPEAN JOURNAL OF PHYSICS, ISSN 1895-1082, Vol. 10, no 5, p. 1038-1053Article in journal (Refereed)
    Abstract [en]

    The problem of proton-antiproton motion in the H- system is investigated by means of the variational method. We introduce a modified nuclear interaction through mass-scaling of the Born-Oppenheimer potential. This improved treatment of the interaction includes the nondivergent part of the otherwise divergent adiabatic correction and shows the correct threshold behaviour. Using this potential we calculate the vibrational energy levels with angular momentum 0 and 1 and the corresponding nuclear wave functions, as well as the S-wave scattering length. We obtain a full set of all bound states together with a large number of discretized continuum states that might be utilized in variational four-body calculations. The results of our calculations gives an indication of resonance states in the hydrogen-antihydrogen system.

  • 182.
    Stenrup, Michael
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Galvan, Ignacio Fdez.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Constrained numerical gradients and composite gradients: Practical tools for geometry optimization and potential energy surface navigation2015In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 36, no 22, p. 1698-1708Article in journal (Refereed)
    Abstract [en]

    A method is proposed to easily reduce the number of energy evaluations required to compute numerical gradients when constraints are imposed on the system, especially in connection with rigid fragment optimization. The method is based on the separation of the coordinate space into a constrained and an unconstrained space, and the numerical differentiation is done exclusively in the unconstrained space. The decrease in the number of energy calculations can be very important if the system is significantly constrained. The performance of the method is tested on systems that can be considered as composed of several rigid groups or molecules, and the results show that the error with respect to conventional optimizations is of the order of the convergence criteria. Comparison with another method designed for rigid fragment optimization proves the present method to be competitive. The proposed method can also be applied to combine numerical and analytical gradients computed at different theory levels, allowing an unconstrained optimization with numerical differentiation restricted to the most significant degrees of freedom. This approach can be a practical alternative when analytical gradients are not available at the desired computational level and full numerical differentiation is not affordable. (c) 2015 Wiley Periodicals, Inc.

  • 183. Stålring, Jonna
    et al.
    Bernhardsson, Anders
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Analytical gradients of a state average MCSCF state and a state average diagnostic2001In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 99, no 2, p. 103-114Article in journal (Refereed)
    Abstract [en]

    An efficient method for calculating the Lagrange multipliers and the analytical gradients of one state included in a state average MCSCF wave function is presented. It is demonstrated that the state average energy of an ‘equal-weight’ scheme is invariant to rotations within the state average subspace and that the corresponding rotations should be eliminated from the Lagrangian equations. Finally, a diagnostic is presented, which gauges the energy difference between a state defined by a state average calculation and the corresponding fully variational multi-configurational SCF state.

  • 184. Stålring, Jonna
    et al.
    Gagliardi, Laura
    Malmqvist, Per-Åke
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    A theoretical study of the 2(1)A(g)<- 1(1)A(g) two-photon transition and its vibronic band in trans-stilbene2002In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 100, no 11, p. 1791-1796Article in journal (Refereed)
    Abstract [en]

    The two-photon spectrum of the 2(1)A(g)–>1(1)A(g) transition in trans-stilbene has been calculated at the complete active space self-consistent field (CASSCF) level of theory. Energies were obtained at the complete active space second-order perturbation (CASPT2) level of theory, while the geometries of both the initial and final states were optimized at the CASSCF level. The energy and the geometry optimizations were performed using an active space of 14 electrons in 14 active pi orbitals. The vibrational frequencies of both states and the two-photon transition (TPT) cross-section were calculated with a smaller active space where the two lowest pi orbitals were kept inactive. A newly implemented algorithm, in the quantum chemical package Molcas was used to determine the two-photon transition intensity. This method requires only the linear response of the CASSCF wavefunction. Furthermore, the vibronic structure of this TPT was studied. The Franck-Condon factors were obtained by calculating the overlap between the vibrational states involved, which were determined from the force fields of both the initial and final states, at the CASSCF level of theory. The results are in agreement with experiment.

  • 185. Stålring, Jonna
    et al.
    Schütz, Martin
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Karlström, Gunnar
    Widmark, Per-Olof
    The ammonia dimer equilibrium dissociation energy: convergence to the basis set limit at the correlated level2002In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 100, no 21, p. 3389-3399Article in journal (Refereed)
    Abstract [en]

    The low-energy region of the intermolecular potential energy hypersurface (PES) of the ammonia dimer was studied at the level of second-order Moller-Plesset perturbation theory (MP2) using a very large basis set. Individual minima were located on the PES employing the counterpoise (CP) correction to account for the basis set superposition error (BSSE). Apart from these canonical MP2 calculations local MP2 (LMP2) calculations were performed. For the latter the BSSE at the correlated level is inherently absent by virtue of the local truncation of the virtual space. Results from canonical and local MP2 calculations are compared and the reliability of the LMP2 method for intermolecular complexes and clusters is discussed. The canonical MP2 calculations predicted five minimum structures, the four most stable ones lying energetically very close. For these four structures single point MP2 energy calculations with a further extended basis set (1024 functions for the ammonia dimer) were performed. The equilibrium dissociation energies so obtained are close to the one-particle basis set limit, as illustrated by a remaining BSSE of less than 0.2 kJ mol(-1). The geometry optimizations at the LMP2 level, using the three most stable canonical MP2 structures as initial geometries, all collapsed to a single minimum corresponding to an asymmetric structural arrangement. A canonical MP2 single point calculation, at that geometry, revealed that the LMP2 minimum structure is virtually as stable as the lowest minima on the canonical MP2 PES. Based on these calculations the global minimum of the ammonia dimer was assigned to a part of the PES represented by an asymmetric structure with an equilibrium dissociation energy of 13.5 +/- 0.3 kJ mol(-1.)

  • 186. Swerts, Ben
    et al.
    Chibotaru, Liviu F.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Seijo, Luis
    Barandiaran, Zoila
    Clima, Sergiu
    Pierloot, Kristin
    Hendrickx, Marc F. A.
    Embedding fragment ab initio model potentials in CASSCF/CASPT2 calculations of doped solids: Implementation and applications2008In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 4, no 4, p. 586-594Article in journal (Refereed)
    Abstract [en]

    In this article, we present a fragment model potential approach for the description of the crystalline environment as an extension of the use of embedding ab initio model potentials (AIMPs). The biggest limitation of the embedding AIMP method is the spherical nature of its model potentials. This poses problems as soon as the method is applied to crystals containing strongly covalently bonded structures with highly nonspherical electron densities. The newly proposed method addresses this problem by keeping the full electron density as its model potential, thus allowing one to group sets of covalently bonded atoms into fragments. The implementation in the MOLCAS 7.0 quantum chemistry package of the new method, which we call the embedding fragment ab inito model potential method (embedding FAIMP), is reported here, together with results of CASSCF/CASPT2 calculations. The developed methodology is applied for two test problems: (i) the investigation of the lowest ligand field states (2)A(1) and B-2(1) of the Cr(V) defect in the YVO4 crystal and (ii) the investigation of the lowest ligand field and ligand-metal charge transfer (LMCT) states at the Mn(II) substitutional impurity doped into CaCO3. Comparison with similar calculations involving AIMPs for all environmental atoms, including those from covalently bounded units, shows that the FAIMP treatment of the YVO4 units surrounding the CrO43- cluster increases the excitation energy B-2(1) -> (2)A(1) by ca. 1000 cm(-1) at the CASSCF level of calculation. In the case of the Mn(CO3)(6)(10-) cluster, the FAIMP treatment of the CO32- units of the environment give smaller corrections, of ca. 100 cm(-1), for the ligand-field excitation energies, which is explained by the larger ligands of this cluster. However, the correction for the energy of the lowest LMCT transition is found to be ca. 600 cm(-1) for the CASSCF and ca. 1300 cm(-1) for the CASPT2 calculation.

  • 187. Söderhjelm, P.
    et al.
    Krogh, J W
    Karlström, G
    Ryde, U
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Accuracy of distributed multipoles and polarizabilities: Comparison between the LoProp and MpProp models2007In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 28, no 6, p. 1083-1090Article in journal (Refereed)
    Abstract [en]

    Localized multipole moments up to the fifth moment as well as localized dipole polarizabilities are calculated with the MpProp and the newly developed LoProp methods for a total of 20 molecules, predominantly derived from amino acids. A comparison of electrostatic potentials calculated from the multipole expansion obtained by the two methods with ab initio results shows that both methods reproduce the electrostatic interaction with an elementary charge with a mean absolute error of similar to 1.5 kJ/mol at contact distance and less than 0.1 kJ/mol at distances 2 angstrom further out when terms up to the octupole moments are included. The polarizabilities are tested with homogenous electric fields and are-found to have similar accuracy. The MpProp method gives better multipole moments unless diffuse basis sets are used, whereas LoProp gives better polarizabilities.

  • 188.
    Sörensen, Lasse Kragh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry. Uppsala Univ, Uppsala Ctr Computat Chem, S-75105 Uppsala, Sweden..
    Lundberg, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Gauge origin independence in finite basis sets and perturbation theory2017In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 683, p. 536-542Article in journal (Refereed)
    Abstract [en]

    We show that origin independence in finite basis sets for the oscillator strengths is possibly in any gauge contrary to what is stated in literature. This is proved from a discussion of the consequences in perturbation theory when the exact eigenfunctions and eigenvalues to the zeroth order Hamiltonian H-0 cannot be found. We demonstrate that the erroneous conclusion for the lack of gauge origin independence in the length gauge stems from not transforming the magnetic terms in the multipole expansion leading to the use of a mixed gauge. Numerical examples of exact origin dependence are shown.

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  • 189.
    Sørensen, Lasse Kragh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Guo, Meiyuan
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lundberg, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Applications to metal K pre-edges of transitionmetal dimers illustrate the approximate origin independence for the intensities in the length representation2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 1-2, p. 174-189Article in journal (Refereed)
    Abstract [en]

    X-ray absorption spectroscopy (XAS) in the metal K pre-edge is a standard probe of electronic and geometric structure of transition metal complexes. Simulating the K pre-edge spectra requires contributions beyond the electric dipole, but if that term is non-zero, the second-order terms, e. g. electric quadrupoles, are no longer origin-independent. In the velocity representation, complete origin independence can be achieved by including all terms to the same order in the oscillator strength. Here, we implement that approach in the length representation and use it for restricted active space (RAS) simulations of metal K pre-edges of iron monomers and dimers. Complete origin independence is not achieved and the size of the remaining errors depends on the electric dipole oscillator strength and its ratio in length and velocity representations. The error in the origin independence is in the ANO basis sets two orders of magnitude smaller than the value of the individual contributions. For systemswith strong electric dipole contributions, the errors are not significant within 3 angstrom from a metal centre, far enough to handlemany multi-metal systems. Furthermore, we discuss the convergence of the multipole expansion, the possibility to assign spectral contributions, and the origin of negative absorption intensities. [GRAPHICS]

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  • 190.
    Sørensen, Lasse Kragh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Kieri, Emil
    Institut für Numerische Simulation, Wegelerstrasse 6, Bonn, Germany.
    Srivastav, Shruti
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Lundberg, Marcus
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Implementation of a semiclassical light-matter interaction using the Gauss–Hermite quadrature: A simple alternative to the multipole expansion2019In: Physical Review A: covering atomic, molecular, and optical physics and quantum information, ISSN 2469-9926, E-ISSN 2469-9934, Vol. 99, no 1, article id 013419Article in journal (Refereed)
  • 191.
    Tamayo-Mendoza, Teresa
    et al.
    Harvard Univ, Dept Chem & Chem Biol, 12 Oxford St, Cambridge, MA 02138 USA.
    Kreisbeck, Christoph
    Harvard Univ, Dept Chem & Chem Biol, 12 Oxford St, Cambridge, MA 02138 USA.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Aspuru-Guzik, Alaprimen
    Harvard Univ, Dept Chem & Chem Biol, 12 Oxford St, Cambridge, MA 02138 USA;Canadian Inst Adv Res, Toronto, ON M5G 1Z8, Canada.
    Automatic Differentiation in Quantum Chemistry with Applications to Fully Variational Hartree-Fock2018In: ACS CENTRAL SCIENCE, ISSN 2374-7943, Vol. 4, no 5, p. 559-566Article in journal (Refereed)
    Abstract [en]

    Automatic differentiation (AD) is a powerful tool that allows calculating derivatives of implemented algorithms with respect to all of their parameters up to machine precision, without the need to explicitly add any additional functions. Thus, AD has great potential in quantum chemistry, where gradients are omnipresent but also difficult to obtain, and researchers typically spend a considerable amount of time finding suitable analytical forms when implementing derivatives. Here, we demonstrate that AD can be used to compute gradients with respect to any parameter throughout a complete quantum chemistry method. We present DiffiQult, a Hartree-Fock implementation, entirely differentiated with the use of AD tools. DiffiQult is a software package written in plain Python with minimal deviation from standard code which illustrates the capability of AD to save human effort and time in implementations of exact gradients in quantum chemistry. We leverage the obtained gradients to optimize the parameters of one-particle basis sets in the context of the floating Gaussian framework.

  • 192.
    Ugandi, Mihkel
    et al.
    Uppsala Univ, Dept Chem Angstrom, Theoret Chem Programme, Box 538, S-75121 Uppsala, Sweden..
    Galvan, Ignacio Fdez
    Uppsala Univ, Dept Chem Angstrom, Theoret Chem Programme, Box 538, S-75121 Uppsala, Sweden..
    Widmark, Per-Olof
    Lund Univ, Dept Theoret Chem, Lund, Sweden..
    Lindh, Roland
    Uppsala Univ, Dept Chem Angstrom, Theoret Chem Programme, Box 538, S-75121 Uppsala, Sweden..
    Uncontracted basis sets for ab initio calculations of muonic atoms and molecules2018In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 118, no 21, article id e25755Article in journal (Refereed)
    Abstract [en]

    In this work, we investigated muonic atoms and molecules from a quantum chemist's viewpoint by incorporating muons in the CASSCF model. With the aim of predicting muonic X-ray energies, primitive muonic basis sets were developed for a selection of elements. The basis sets were then used in CASSCF calculations of various atoms and molecules to calculate muonic excited states. We described the influence of nuclear charge distribution in predicting muonic X-ray energies. Effects of the electronic wave function on the muonic X-ray energies were also examined. We have computationally demonstrated how the muon can act as a probe for the nuclear charge distribution or electronic wave function by considering lower or higher muonic excited states, respectively.

  • 193.
    Vacher, Morgane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Brakestad, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Karlsson, Hans O.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Dynamical Insights into the Decomposition of 1,2-Dioxetane2017In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 13, no 6, p. 2448-2457Article in journal (Refereed)
    Abstract [en]

    Chemiluminescence in 1,2-dioxetane occurs through a thermally activated decomposition reaction into two formaldehyde molecules. Both ground-state and nonadiabatic dynamics (including singlet excited states) of the decomposition reaction have been simulated, starting from the first O–O bond-breaking transition structure. The ground-state dissociation occurs between t = 30 fs and t = 140 fs. The so-called entropic trap leads to frustrated dissociations, postponing the decomposition reaction. Specific geometrical conditions are necessary for the trajectories to escape from the entropic trap and for dissociation to be possible. The singlet excited states participate as well in the trapping of the molecule: dissociation including the nonadiabatic transitions to singlet excited states now occurs from t = 30 fs to t = 250 fs and later. Specific regions of the seam of the So/S1 conical intersections that would "retain" the molecule for longer on the excited state have been identified.

  • 194.
    Vacher, Morgane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Farahani, Pooria
    Univ Sao Paulo, Inst Quim, Dept Quim Fundamental, BR-05508000 Sao Paulo, Brazil..
    Valentini, Alessio
    Univ Liege, Dept Chim, Allee 6 Aout 11, B-4000 Liege, Belgium..
    Frutos, Luis Manuel
    Univ Alcala De Henares, Dept Quim Fis, E-28871 Madrid, Spain..
    Karlsson, Hans O.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    How Do Methyl Groups Enhance the Triplet Chemiexcitation Yield of Dioxetane?2017In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 8, no 16, p. 3790-3794Article in journal (Refereed)
    Abstract [en]

    Chemiluminescence is the emission of light as a result of a nonadiabatic chemical reaction. The present work is concerned with understanding the yield of chemiluminescence, in particular how it dramatically increases upon methylation of 1,2-dioxetane. Both ground-state and nonadiabatic dynamics (including singlet excited states) of the decomposition reaction of various methyl-substituted dioxetanes have been simulated. Methyl-substitution leads to a significant increase in the dissociation time scale. The rotation around the O-C-C-O dihedral angle is slowed; thus, the molecular system stays longer in the "entropic trap" region. A simple kinetic model is proposed to explain how this leads to a higher chemiluminescence yield. These results have important implications for the design of efficient chemiluminescent systems in medical, environmental, and industrial applications.

  • 195.
    Vacher, Morgane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Fernández Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Ding, Bo-Wen
    Schramm, Stefan
    Berraud-Pache, Romain
    Naumov, Pance
    Ferré, Nicolas
    Liu, Ya-Jun
    Navizet, Isabelle
    Roca-Sanjuán, Daniel
    Baader, Wilhelm J.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Chemi- and Bioluminescence of Cyclic Peroxides2018In: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 118, no 15, p. 6927-6974Article, review/survey (Refereed)
    Abstract [en]

    Bioluminescence is a phenomenon that has fascinated mankind for centuries. Today the phenomenon and its sibling, chemiluminescence, have impacted society with a number of useful applications in fields like analytical chemistry and medicine, just to mention two. In this review, a molecular-orbital perspective is adopted to explain the chemistry behind chemiexcitation in both chemi- and bioluminescence. First, the uncatalyzed thermal dissociation of 1,2-dioxetane is presented and analyzed to explain, for example, the preference for triplet excited product states and increased yield with larger nonreactive substituents. The catalyzed fragmentation reaction and related details are then exemplified with substituted 1,2-dioxetanone species. In particular, the preference for singlet excited product states in that case is explained. The review also examines the diversity of specific solutions both in Nature and in artificial systems and the difficulties in identifying the emitting species and unraveling the color modulation process. The related subject of excited-state chemistry without light absorption is finally discussed. The content of this review should be an inspiration to human design of new molecular systems expressing unique light-emitting properties. An appendix describing the state-of-the-art experimental and theoretical methods used to study the phenomena serves as a complement.

  • 196.
    Vacher, Morgane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Galván, Ignacio
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Brakestad, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Karlsson, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Non-adabatic dynamics of the 1,2-dioxetane chemiluminescence2017In: Abstracts of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 254Article in journal (Other academic)
  • 197. Vancoillie, Steven
    et al.
    Delcey, Mickaël G.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Vysotskiy, Victor
    Malmqvist, Per-Ake
    Veryazov, Valera
    Parallelization of a multiconfigurational perturbation theory2013In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 34, no 22, p. 1937-1948Article in journal (Refereed)
    Abstract [en]

    In this work, we present a parallel approach to complete and restricted active space second-order perturbation theory, (CASPT2/RASPT2). We also make an assessment of the performance characteristics of its particular implementation in the Molcas quantum chemistry programming package. Parallel scaling is limited by memory and I/O bandwidth instead of available cores. Significant time savings for calculations on large and complex systems can be achieved by increasing the number of processes on a single machine, as long as memory bandwidth allows, or by using multiple nodes with a fast, low-latency interconnect. We found that parallel efficiency drops below 50% when using 8-16 cores on the shared-memory architecture, or 16-32 nodes on the distributed-memory architecture, depending on the calculation. This limits the scalability of the implementation to a moderate amount of processes. Nonetheless, calculations that took more than 3 days on a serial machine could be performed in less than 5 h on an InfiniBand cluster, where the individual nodes were not even capable of running the calculation because of memory and I/O requirements. This ensures the continuing study of larger molecular systems by means of CASPT2/RASPT2 through the use of the aggregated computational resources offered by distributed computing systems.

  • 198. Veryazov, Valera
    et al.
    Widmark, Per-Olof
    Serrano-Andrés, Luis
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Roos, Björn O
    MOLCAS as a development platform for quantum chemistry software2004In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 100, no 4, p. 626-635Article in journal (Refereed)
    Abstract [en]

    This work presents the quantum chemistry package MOLCAS, with emphasis on its usefulness as a platform for developing new quantum chemical codes, and the reader is assumed to be familiar with such a process. The development of new codes for quantum chemistry is a time-consuming job that can be dramatically simplified by using libraries for standard problems (such as calculation of integrals), and tools to surmount computer language and operating system limitations. The MOLCAS quantum chemistry software contains modules for a variety of quantum chemical methods, such as Hartree-Fock (HF), density functional theory (DFT), coupled-cluster (CC), and multiconfigurational (MCSCF) approaches, including second-order perturbation theory. It runs on almost all UNIX-like platforms and contains a single source code for 32- and 64-bit architecture, as well as for serial and parallel execution. The MOLCAS environment allows the user to include, in a simple way, new codes integrated with libraries and other MOLCAS modules. The key features of the MOLCAS environment include a modular structure, an easy-to-use environment, an application programming interface (API) library for solving common computational and system-oriented problems, a solid verification control system, and tools for a distributed development.

  • 199. Wang, Xuefeng
    et al.
    Andrews, Lester
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Veryazov, Valera
    Roos, Björn O
    A combined theoretical and experimental study of simple terminal group 6 nitride and phosphide N MX3 and P MX3 molecules2008In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, no 35, p. 8030-8037Article in journal (Refereed)
    Abstract [en]

    Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF3, PF3, and PCl3 to form the simple lowest energy N MF3, and P MX3 products following insertion and halogen transfer, with the exception of P CrF3, which is a higher energy species and is not observed. The E MX3 pnictide metal trihalide molecules are identified from both argon and neon matrix infrared spectra and frequencies calculated by density functional theory and multiconfigurational second-order perturbation theory (CASSCF/CASPT2). These simple terminal nitrides involve strong triple bonds, which range from 2.80 to 2.77 to 2.59 natural bond order for M = W, Mo, and Cr, respectively, as computed by CASSCF/CASPT2, and the M N stretching frequencies also follow this order. The terminal phosphides are weaker with bond orders 2.74, 2.67, and 2.18, respectively, as the more diffuse 3p orbitals are less effective for bonding to the more compact metal valence d orbitals.

  • 200.
    Yanai, Takeshi
    et al.
    Inst Mol Sci, Dept Theoret & Computat Mol Sci, Okazaki, Aichi, Japan.;Grad Univ Adv Studies, Sch Phys Sci, Dept Funct Mol Sci, Okazaki, Aichi, Japan..
    Kurashige, Yuki
    Kobe Univ, Grad Sch Syst Informat, Kobe, Hyogo, Japan.;PRESTO, Japan Sci & Technol Agcy, Kawaguchi, Saitama, Japan.;Kyoto Univ, Grad Sch Sci, Dept Chem, Kyoto, Japan..
    Saitow, Masaaki
    Grad Univ Adv Studies, Sch Phys Sci, Dept Funct Mol Sci, Okazaki, Aichi, Japan.;Max Planck Inst Chem Energiekonvers, Stiftstr 34-36, D-45470 Mulheim, Germany..
    Chalupsky, Jakub
    Acad Sci Czech Republ, Inst Organ Chem & Biochem, Prague, Czech Republic..
    Lindh, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
    Malmqvist, Per-Åke
    Lund Univ, Dept Theoret Chem, Lund, Sweden..
    Influence of the choice of projection manifolds in the CASPT2 implementation2017In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 115, no 17-18, p. 2077-2085Article in journal (Refereed)
    Abstract [en]

    The Complete Active Space Second-Order Perturbation Theory (CASPT2) is well-established as a high-accuracy electronic structure method. It was originally implemented in the early 1990s to an efficient computer code in the molcas program suite, and this implementation has been extensively used as a standard tool. Here, we report a comparison of it against two independent computer-aided implementations of the CASPT2 method, revealing that the CASPT2 energies provided by the original code of molcas (version 8 or earlier) are inconsistent with the predictions of the lately developed computer-aided implementations. It is shown that this error is associated with the projections of the first-order equation onto the fully internally contracted multireference bases which are partially inconsistent between the left- and right-hand sides. The degree of the errors is assessed by performing illustrative CASPT2 calculations. The errors in total CASPT2 energies are demonstrated to be negligible relative to chemical accuracy in many cases, while there is a difficult case where they may substantially alter chemical description. The incorporation of the consistent projections into molcas has been carried out, which is available in the version 8 sp1.

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