CHARGE-OVERLAP MODEL OF PHYSICAL INTERACTIONS AND A COMBINING RULE FOR UNLIKE SYSTEMS

We consider the hypothesis that the repulsive interaction V in van der Waalstype systems is proportional to the charge-density overlap integral S. This hypothesis is shown to be accurate for interatomic interactions, with a proportionality coefficient V/S10 a.u. in most cases. It is related to previous results of Kim, Kim, and Lee [Chem. Phys. Lett. 80, 574 (1971)] and Nyeland and Toennies [Chem. Phys. Lett. 127, 172 (1986)]. The hypothesis is consistent with the effective-medium theory result for immersion of an atom or H2 into jellium. For the interatomic problem, the computed values of the decay lengths (-1) of V are simply related to those (-1) of the asymptotic atomic charge density. This model provides a theoretical basis for an improved combining rule for the minimum position RAB of the total interaction Vtot between unlike species. It also explains why this quantity depends systematically on the ionization ratio IB/IA of the interacting species.