Quantum theoretical calculations of van der Waals interactions between molecules. Anisotropic long range interactions

Abstract

AbstractsPresenting a relatively simple ab initio method to calculate full van der Waals interaction potentials between molecules, we give rules for the optimization of basis functions which permit the reliable evaluation of second order long range interactions. Closed expressions for the long range interaction energy are derived in which the orientational dependence is simplified to the utmost. Calculations show that even for molecules which have no dipole moment, such as ethylene, the strongly anisotropic electrostatic interactions are of the same magnitude as the dispersion interactions, but also that the anisotropic ("cross") terms in the dispersion energy are about equal in size to the corresponding "quadratic" terms. Even though these anisotropic forces cancel to a large extent in the cohesion energy of the ethylene crystal, they can have important effects on some of the other crystal properties.Nous présentons une méthode ah initio relativement simple pour calculer des potentiels complets représentant l'interaction de type van der Waals entre des molécules, et nous donnons des règles pour l'optimisation des fonctions de base, qui permettent une évaluation correcte des interactions à longue portée du second ordre. On obtient des expressions analytiques finies pour l'énergie d 'interaction à longue portée, dans lesquelles la dépendance orientationnelle est simplifiée jusqu'à l'extrême. Des calculs montrent, que même pour des molécules comme l'éthylène, qui n'ont pas de moment dipolaire, les interactions électrostatiques fortement anisotropes sont du même ordre de grandeur que les interactions de dispersion, mais aussi que les termes anisotropes de l'énergie de dispersion ("cross terms") sont presqu'égaux aux termes "carrés" correspondants. Même si ces forces anisotropes se compensent en grand dans l'énergie de cohésion du cristal d 'éthylène, elles peuvent avoir des effects importants sur d'autres propriétés cristallines.

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