Many-body forces and stability of the alkaline-earth tetramers

Abstract

The comparative study of the interaction energy and its many-body decomposition for Be 4, Mg 4, and Ca 4 at the all-electron CCSD(T)/aug-cc-pVQZ level is performed. For study of dependence of the binding energy and the orbital population on the cluster size the corresponding dimers and trimers were also calculated at the same level of theory. In comparison with weakly bound dimers, the binding energy in trimers and, especially, in tetramers drastically increases; e.g., E b / N in Be 3 is 7 times larger and in Be 4 is 18.4 times larger than in Be 2. This sharp increase is explained as a manifestation of many-body forces. As follows from the many-body decomposition, the tetramers, and trimers as well, are stabilized by the three-body forces, whereas the two- and four-body forces are repulsive. The attractive contribution to the three-body forces has a three-atom electron exchange origin. The latter benefits the promotion of ns-electrons to np-orbitals. The natural bond orbital (NBO) population analysis reveals a relatively large np-population in trimers and tetramers (in Be 4 it is equal to 2p 0.44). The population of the valence np-orbitals leads to the sp-hybridization providing the covalent bonding.