Combination Rules for van der Waals Force Constants

A family of combination rules, from which van der Waals coefficients for interactions between unlike molecules may be estimated in terms of those for the corresponding like interactions and the static polariabilities, is tested empirically and related to an approximate parameterization of the frequency-dependent polarizability function. The treatment includes two- and three-body dipole force constants and the higher-multipole terms of the second-order two-body dispersion energy. The rules give agreement within a few percent (about the same accuracy as the input data) when compared with accurate unlike coefficients for systems involving H, He, Ne, Ar, Kr, Xe, Li, Na, K, Rb, Cs, Hg, H2, N2, CH4, He(21S), He(23S), He+, Li+, Na+, K+, Rb+, Cs+. Knowledge of n like coefficients and polarizabilities permits calculation of n(n − 1) / 2 unlike two-body and n(n − 1)(n + 4) / 6 unlike three-body force constants.