A simple theoretical model for the van der Waals potential at intermediate distances. III. Anisotropic potentials of Ar–H2, Kr–H2, and Xe–H2

Abstract

A simple semiclassical theory of the van der Waals potential, which uses no empirical constants, was proposed and tested on atom–atom systems in an earlier paper [J. Chem. Phys. 66, 1496 (1977)]. In a following paper [J. Chem. Phys. 68, 5501 (1978)] it was successfully applied to the prediction of the radial v0(R) and anisotropic parts v2(R) of the van der Waals potential expanded as V(R,γ)=v0(R)+v2)R)× P2(cos γ) (R is the distance between centers of mass and γ is the angle between R and the molecular axis) of He–H2 and Ne–H2 for which all the necessary ab initio input data were available. In the present paper the method is applied to the heavier systems Ar–H2, Kr–H2, and Xe–H2 for which the ab initio data are not available. The dispersion terms were estimated using the precise combining rule described in II. The anisotropic Born–Mayer parameters for the repulsive potentials were estimated by successive application of the Gilbert–Smith combining rules. The necessary input was the ab initio repulsive anisotropy in He–H2 and the repulsive parameters for Ar–Ar, Kr–Kr, and Xe–Xe, which could be estimated from the experimental potential well parameters. The resulting Born–Mayer parameters were adjusted slightly to agree with the experimental v0 potential parameters. The v2 potentials predicted in this way are presented and compared with the best available experimental potentials of Le Roy and co-workers and Zandee and Reuss. Cross section anisotropy factors have also been calculated for direct comparison with the measured orientation dependence of integral cross sections measured by Reuss and co-workers. For all systems the agreement with experiment is very good and within the experimental error. Finally, a new law of corresponding states which predicts that the reduced shapes of the spherical symmetric and anisotropy potentials are identical is proposed.