An overlap model for estimating the anisotropy of repulsion

Abstract

There is an urgent need for accurate anisotropic site-site intermolecular pair potentials for use in realistic simulations of the condensed phases and spectra of van der Waals molecules. However, all attempts to find an analytical form for the repulsion energy have relied on empirical models with many fitted parameters, whose determination requires large quantities of accurate experimental or ab initio data. In this paper, we develop and assess a procedure for estimating the repulsion anisotropy from the monomer wavefunctions which is straightforward, computationally inexpensive and capable of high accuracy. The method is based on the observation that the intermolecular repulsion energy is closely related to the overlap between the unperturbed charge densities of the interacting molecules. The overlap can be treated analytically, leading to an anisotropic site-site functional form. Model pair repulsion potentials with two sites are obtained for (F₂)₂ and (Cl₂)₂ by ignoring the bonding charge density. For (N₂)₂ a novel method of distributing the bonding charge density onto the atoms and the bond centre is used to obtain a three-site model, and this technique is also applied to (F₂)₂ and (Cl₂)₂. The resulting pair potentials, which have one or two fitted parameters, are found to reproduce an ab initio repulsion surface much more accurately than an isotropic atom-atom potential with two parameters. Since the method uses only the monomer charge densities, it can be extended to larger molecules. The resulting repulsion potential is suitable for incorporation into a complete model of the interaction energy.