The effect of anisotropic higher order dispersive interactions on the conformational stabilities of argon clusters

Abstract

The point–dipole interaction model of polarizability developed by Applequist is applied to a theory of long-range intermolecular dispersive forces to explore the effect of anisotropic many body interactions on the properties of various van der Waals complexes. Our anisotropic point–dipole interaction formalism appears to be significantly more accurate than isotropic pairwise formalisms at medium to large internuclear separations. At short internuclear distances, however, our procedures overestimate the dispersion energy due to the inherent neglect of electron exchange effects. Nevertheless, for large clusters of atoms or molecules, the present formalism provides a dependable and relatively rapid calculational approach to the analysis of many body dispersion forces. The only-time consuming calculation of the entire procedure is the inversion of a 3N×3N matrix, where N is the number of atoms in the van der Waals complex. Potential energy surfaces for three noble gas atoms are predicted to have minima for both the triangular and linear configurations. Isotropic pairwise calculations predict a minimum only for the triangular configuration. Three other types of clusters are also investigated: a linear array, a planar triangular lattice, and a hexagonal closest packed cluster. The typical effect of the anisotropic many body interactions is to lower the calculated dispersion energy by 5%–20%. The magnitude of the energy lowering is approximately proportional to the optical anisotropy of the system. Our results imply that anisotropic effects are only important when the optical anisotropy is larger than 0.001.