An improved simple model for the van der Waals potential based on universal damping functions for the dispersion coefficients

Abstract

Starting from our earlier model [J. Chem. Phys. 66, 1496 (1977)] a simple expression is derived for the radial dependent damping functions for the individual dispersion coefficients C2n for arbitrary even orders 2n. The damping functions are only a function of the Born–Mayer range parameter b and thus can be applied to all systems for which this is known or can be estimated. For H(1S)–H(1S) the results are in almost perfect agreement with the very accurate recent ab initio damping functions of Koide, Meath, and Allnatt. Comparisons with less accurate previous calculations for other systems also show a satisfactory agreement. By adding a Born–Mayer repulsive term [A exp(−bR)] to the damped dispersion potential, a simple universal expression is obtained for the well region of the atom–atom van der Waals potential with only five essential parameters A, b, C6, C8, and C10. The model has been tested for the following representative systems: H2 3Σ, He2, and Ar2 as well as NaK 3Σ and LiHg, which include four chemically different types of van der Waals interactions for which either very precise theoretical or experimental data is available. For each system the ab initio dispersion coefficients together with the well-known parameters ε and Rm were used to determine A and b from the model potential. With these values the reduced potentials were calculated and found to agree with the experimental potentials to better than 1% and always less than the experimental uncertainties. Some of the implications of the new model are discussed.