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 Be-n, Mg-n, and Ca-n (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. (C) 2000 American Institute of Physics. [S0021- 9606(00)30333-6].