Nature of binding in the alkaline–earth clusters: Be3, Mg3, and Ca3

The study of the interaction energy and its many-body decomposition in a broad distance interval for the Ben, Mgn, and Can (n=2,3) clusters at the SCF and MP4 levels are performed. A comparative analysis of the obtained results allows one to conclude that the only stabilization factor in the dimers is the dispersion forces. So, the alkaline–earth dimers can be attributed to the van der Waals molecules. The trimers are stabilized by the two-body localized dispersion forces and three-body delocalized exchange forces. The binding in the alkaline–earth trimers has a mixed physical (van der Waals) and chemical (nonadditive exchange) nature. An NBO population analysis reveals a relatively large p-population in all clusters. A surprisingly large p-population at the MP4 level is also obtained for the isolated atoms.