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Restricted active space calculations of L-edge X-ray absorption spectra: From molecular orbitals to multiplet states
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.ORCID iD: 0000-0002-3607-3995
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.ORCID iD: 0000-0001-9883-3569
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Theoretical Chemistry.
Stockholm Univ, AlbaNova Univ Ctr, Dept Phys, SE-10691 Stockholm, Sweden.
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2014 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 141, no 12, 124116Article in journal (Refereed) Published
Abstract [en]

The metal L-edge (2p -> 3d) X-ray absorption spectra are affected by a number of different interactions: electron-electron repulsion, spin-orbit coupling, and charge transfer between metal and ligands, which makes the simulation of spectra challenging. The core restricted active space (RAS) method is an accurate and flexible approach that can be used to calculate X-ray spectra of a wide range of medium-sized systems without any symmetry constraints. Here, the applicability of the method is tested in detail by simulating three ferric (3d(5)) model systems with well-known electronic structure, viz., atomic Fe3+, high-spin [FeCl6](3-) with ligand donor bonding, and low-spin [Fe(CN)(6)](3-) that also has metal backbonding. For these systems, the performance of the core RAS method, which does not require any system-dependent parameters, is comparable to that of the commonly used semi-empirical charge-transfer multiplet model. It handles orbitally degenerate ground states, accurately describes metal-ligand interactions, and includes both single and multiple excitations. The results are sensitive to the choice of orbitals in the active space and this sensitivity can be used to assign spectral features. A method has also been developed to analyze the calculated X-ray spectra using a chemically intuitive molecular orbital picture.

Place, publisher, year, edition, pages
2014. Vol. 141, no 12, 124116
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-236075DOI: 10.1063/1.4896373ISI: 000342844100021PubMedID: 25273421OAI: oai:DiVA.org:uu-236075DiVA: diva2:762700
Note

Correction in: Journal of Chemical Physics, vol. 141, issue 4, article number: 149905, DOI: 10.1063/1.4908043 ISI: 000349847000064

Available from: 2014-11-12 Created: 2014-11-12 Last updated: 2016-11-03Bibliographically approved
In thesis
1. Extending the Reach of Accurate Wavefunction Methods
Open this publication in new window or tab >>Extending the Reach of Accurate Wavefunction Methods
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Multiconfigurational quantum chemistry methods, and especially the multiconfigurational self-consistent field (MCSCF) and multireference perturbation theory (MRPT2), are powerful tools, particularly suited to the accurate modeling of photochemical processes and transition metal catalysis. However, they are limited by their high computational cost compared to other methods, especially density functional theory. Moreover, there are areas where they would be expected to perform well, but where they are not applied due to lack of experience.

This thesis addresses those issues. First, the efficiency of the Cholesky decomposition approximation to reduce the cost of MCSCF and MRPT2 without sacrificing their accuracy is demonstrated. This then motivates the extension of the Cholesky approximation to the computation of MCSCF nuclear gradients, thus strongly improving the ability to perform MCSCF non-adiabatic molecular dynamics. Typically, a tenfold speed-up is observed allowing dynamic simulation of larger systems or over longer times.

Finally, multiconfigurational methods are applied to the computation of X-ray spectra of transition metal complexes. The importance of the different parameters in the calculation is systematically investigated, laying the base for wider applications of those accurate methods in the modeling of X-ray spectroscopy. A tool to analyze the resulting spectrum in terms of molecular orbitals is also presented, strengthening the interplay between theory and experiments.

With these developments and other significant ones that have happened in recent years, multiconfigurational methods can now reach new grounds and contribute to important new discoveries

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1228
Keyword
Quantum chemistry, Density fitting, CASSCF, Analytical gradients, Photochemistry, X-ray spectroscopy
National Category
Theoretical Chemistry
Research subject
Chemistry with specialization in Quantum Chemistry
Identifiers
urn:nbn:se:uu:diva-243573 (URN)978-91-554-9168-0 (ISBN)
Public defence
2015-03-31, Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2015-03-10 Created: 2015-02-10 Last updated: 2015-04-14Bibliographically approved
2. Electronic structures of transition metal complexes-core level spectroscopic investigation
Open this publication in new window or tab >>Electronic structures of transition metal complexes-core level spectroscopic investigation
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Uppsala: Uppsala University, The Department of Chemistry The Ångström Laboratory, 2016. 40 p.
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:uu:diva-275074 (URN)
Presentation
2016-02-25, Å64119, Ångström Laboratory, Uppsala, 22:52 (English)
Opponent
Supervisors
Available from: 2016-02-05 Created: 2016-01-28 Last updated: 2016-11-08Bibliographically approved

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