On the Convergence of Multibody Expansions for Short Range Intermolecular Forces. A SCF Calculation of He3 and He4 Systems

Abstract

In this paper we calculate the potential energy surface of He₃ within the SCF—LCAO—MO approximation to test the pairwise additivity of intermolecular forces at short range. We obtain large nonadditive contributions at very close distances, but in general, in the neighborhood of the van der Waals well ($\text{∼5.6}\mathrm{a.u.}$ ), they are negligible. However the study of a few He₄ configurations shows that four‐body effects can be important enough to question the convergence of many‐body expansions.