IS THE ISOTROPIC ATOM-ATOM MODEL POTENTIAL ADEQUATE?

We discuss some of the problems that have frustrated the development of reliable model intermolecular potentials for polyatomic molecules. In particular, the usual assumption of an isotropic atom-atom model potential is analysed, and evidence for its inadequacies is presented. A new approach to designing model potentials, an anisotropic site-site model, is introduced by describing several applications to both small and organic molecules, including molecular dynamics and Monte Carlo simulations. The anisotropy required in an atom-atom potential can be directly linked to the non-spherical features in the valence electron distribution, such as lone pairs and II electrons. An accurate electrostatic model for these effects can be constructed from a distributed multipole analysis of the ab initio wavefunction. The empirically required forms of anisotropy in the repulsion potential can also be qualitatively linked to the molecular electron density difference map. Thus, consideration of the molecular bonding can be a useful indication of how to construct adequate model intermolecular pair potentials.