A simple theoretical model for the van der Waals potential at intermediate distances. IV. The bond distance dependence of the potential hypersurfaces for He–H2 and Ne–H2 also for the repulsive region

A simple theory for the van der Waals potential in the region of the well minimum, which previously has been successfully applied to the prediction of the isotropic atom–atom [J. Chem. Phys. 66, 1496 (1977)] and the anisotropic atom–diatom potentials [J. Chem. Phys. 68, 5501 (1978); 74, 1148 (1981)], has been extended to calculate the full potential hypersurface including the H2 bond distance dependence for He–H2 and Ne–H2. By taking advantage of the known potential parameters in the united atom limits He–He and Ne–He, respectively, the potential hypersurface is predicted over a wide range of bond distances. The model is modified to also provide a good estimate of the true potential in the repulsive region (V≃1 eV). The results for He–H2 are compared with a recently calculated CI type hypersurface [Meyer, Hariharan, and Kutzelnigg, J. Chem. Phys. 73, 1880 (1980)] and found to be in good agreement in the region of intermediate distances of the center of masses (R≈3.0 Å), and for H2 bond distances r in the range (0.50–1.00 Å). The R dependence of the vibrational coupling matrix elements are presented for both systems and the implications for the vibrational relaxation rates of both systems are discussed.