MANY-BODY FORCES AND ELECTRON CORRELATION IN SMALL METAL-CLUSTERS

The many-body decomposition of the interaction energy for Be-N and Li- N (N'=2 to 4) clusters is calculated in two approximations: the self-c onsistent-field method and the Muller-Plesset perturbation theory up t o the fourth order. This allows us to estimate the electron-correlatio n contributions to the many-body forces. The explicit expressions for these contributions in the perturbation theory formalism are obtained. We present a comparative analysis of the role of electron correlation s in the Be-N and Li-N cluster formations and in the many-body interac tions in these clusters. As follows from our results, the contribution of electron correlation to many-body interactions is essential for bo th the Be-N and Li-N clusters, especially for the latter ones, where n onadditivities are surprisingly large.