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-consistent-field method and the Muller-Plesset perturbation theory up to the fourth order. This allows us to estimate the electron-correlation 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 correlations in the Be-N and Li-N cluster formations and in the many-body interactions in these clusters. As follows from our results, the contribution of electron correlation to many-body interactions is essential for both the Be-N and Li-N clusters, especially for the latter ones, where nonadditivities are surprisingly large.