A simple semi-empirical approach to the intermolecular potential of van der Waals systems

Abstract

A simple potential model for the evaluation of isotropic pairwise interaction energies is proposed and used here to predict the lowest triplet state curve of all alkali-alkali interactions. The model, which for the alkali dimers contains no adjustable parameters, expresses the interaction potential in terms of the Hartree-Fock interaction energy, V _HF(R), and the interatomic correlation energy as approximated semi-empirically from the second order dispersion energy calculated including the effect of charge overlap between the electron clouds of the two interacting species, V _inter/disp(R). Lack of repulsive near Hartree-Fock ab initio results for the triplet state of most alkali dimers has been circumvented by proposing a generalized screened Coulomb type repulsion for these systems. The sum of the additional contributions to the interaction energy [V _intra(R) + V _intra-inter(R)], which arise from intra-atomic correlation effects and intra-inter coupling terms, is also shown to be well approximated by introducing a single adjustable parameter (λ) into the model. However, for systems with a small number of effective electrons such as the alkali dimers the atomic correlation error is likely to remain essentially constant for most internuclear distances of interest, thus justifying the approximation made here that [V _intra(R) + V _intra-inter(R)] is zero for these systems (i.e. λ = 1).

Notes

The results of the present work were presented under the title ‘A simple, yet reliable, approach to the lowest triplet state of alkali diatoms’ at the 4th Annual Meeting of the Portuguese Chemical Society, Lisbon, April 1981.