ANALYSIS OF THE INTERMOLECULAR POTENTIAL OF AR-CH4 - AN ABINITIO STUDY

The perturbation theory of intermolecular forces in conjunction with the supermolecular Moller-Plesset treatment is applied in the analysis of the potential energy surface of Ar-CH4. The anisotropy of the Ar-CH4 potential energy surface is chiefly due to exchange repulsion. In the equilibrium structure, Ar approaches the face of CH4 tetrahedron thus avoiding contacts with C-H bonds. The equilibrium Ar-C separation was found equal to 7.5 a0 and the D(e) energy to 113 cm-1. We estimate that our D(e) may be too small by up to 25% with respect to the accurate value. The properties of Ar-CH4 are also compared with other Ar-molecule systems, such as Ar-NH3, Ar-H2O, and Ar-HCl. We find that the equilibrium structures of weak proton donors bound to Ar (CH4, Nh3) are determined by the exchange repulsion, while those of efficient proton donors, such as HCl (and to a lesser extent H2O), result from the strong polarization of Ar in the field of a molecule.