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Dispersion Corrections to the Surface Tension at Planar Surfaces
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.ORCID iD: 0000-0001-8508-6402
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Molecular dynamics simulations are usually performed using cutoffs (rc) for the short-ranged dispersion interactions (r−6). For isotropic systems, long-range interactions are often added in a continuum approximation. This usually leads to excellent results that are independent of the cutoff length down to about 1 nm. For systems with interfaces or other anisotropic systems the situation is more complicated. We study here planar interfaces, focusing on the surface tension, which is sensitive to cutoffs. Previous analytic results giving the long-range correction to the surface tension of a liquid-vapor interface as a two- or three-dimensional integral are revisited. They are generalized by introducing a dispersion density profile which makes it possible to handle multi-component systems. For the simple but common hyperbolic tangent profile the integral may be Taylor-expanded in the dimensionless parameter obtained by dividing the profile width with the cutoff length. This parameter is usually small and excellent agreement with numerical calculations of the integral is obtained by keeping two terms in the expansion. The results are compared to simulations with different lengths of the cutoff for some simple systems. The surface tension in the simulations varies linearly in rc−2, although a small rc−4-term may be added to improve the agreement. The slope of the rc−2-line could in several cases be predicted from the change in dispersion density at the interface. The disagreements observed in some cases when comparing to theory, occur when the finite cutoff used in the simulations causes structural differences compared to long-range cutoffs or Ewald summation for the r−6-interactions. 

National Category
URN: urn:nbn:se:kth:diva-187460OAI: diva2:930348

QC 20160523

Available from: 2016-05-23 Created: 2016-05-23 Last updated: 2016-05-23Bibliographically approved
In thesis
1. Dispersion Corrections at Planar Surfaces
Open this publication in new window or tab >>Dispersion Corrections at Planar Surfaces
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

When simulating a molecular system, a cutoff distance for interactions is often used to speed up the simulations. This is made at the cost of neglecting some interactions which will lead to inaccurate results for energy, pressure components and surface tension (for systems with surfaces). To compensate for the neglected long-range interactions, continuum corrections can be added to the surface tension, system energies and pressures. For a homogenous isotropic system this is straight-forward but for a system with a surface it is more complicated. In this work we have derived expressions for the corrections to the surface tension, system energies and pressures that are more general than previous results. When these corrections are added to multi-component systems with a surface (or single-component systems with vacuum) they compensate for the change in surface tension, system energy and pressures due to the finite cutoff. When simulating systems with no Coulomb-interactions, the structure of the system may change significantly if the cutoffs are too short. If this is the case then these corrections alone will not be enough. The solution is to add corrections to the force acting on each molecule added during the simulation, which we derive in this work. This solves the structural problem at low cutoffs and makes it possible to calculate an accurate surface tension independent of cutoff. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 38 p.
TRITA-FYS, ISSN 0280-316X ; 2016:28
National Category
Research subject
Biological Physics
urn:nbn:se:kth:diva-187462 (URN)978-91-7729-037-7 (ISBN)
2016-06-14, FB55, Albanova University Center, Roslagstullsbacken 21, 106 91 Stockholm, Stockholm, 10:00 (English)
Available from: 2016-05-23 Created: 2016-05-23 Last updated: 2016-05-23Bibliographically approved

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