Charge-Overlap Effects. Dispersion and Induction Forces

For nondegenerate atom–atom and atom–ion interactions the long-range interaction energy is expressed as a sum of “nonexpanded” angular components. If charge overlap is neglected each of the nonexpanded energies goes to a well-defined limit; namely to one of the usual R−1-expanded induction or dispersion energies or else to zero (R is the internuclear separation). The R−1 expansions appear to be asymptotic expansions of the nonexpanded results and are valid only for large R. As specific examples the H–H+ and H–H interactions are considered as model systems for studying charge overlap effects in induction and dispersion forces. The calculations are carried out variationally using the complete discrete Laguerre functions of degree (2l + 2) as a radial basis set. The asymptotic nature of the R−1 expansions for these interactions is discussed by comparing the various expanded and nonexpanded results. The importance of the nonexpanded interaction energies is also discussed briefly in general.