Validity of the multipole results for first-order molecule-molecule interaction energies.

Abstract

The HF-HF, LiH-LiH and N2-N2 interactions, together with previous work on the H2-H2 interaction, are used as models for discussing how charge overlap effects in the first-order Coulomb energy vary as a function of orientation and intermolecular separation as the nature of the interacting species changes. For interactions such as H2-H2 and HF-HF, where the interacting molecules have positive even permanent multipole moments, charge overlap effects are relatively small and the usual R -1 multipole results for the interaction energy between the permanent moments of the interacting molecules give a remarkably good representation of the first-order Coulomb energy until R becomes quite small. For interactions such as LiH-LiH and N2-N2, where the interacting molecules have negative even permanent moments, charge overlap effects are large as a relative function of R and the multipolar results yield a poor representation of the interaction energy even for moderately large values of R. Some of the problems and erro...