The bond nature of alkaline-earth homonuclear metal clusters investigated with pseudopotential CI method

The electronic stucture of Be_l, Mg_m and Ca_n ( 1? 7, m and n ? 5) metal clusters has been investigated by means of an ab initio pseudopotential method followed by multireference double-excitation configuration interaction (PP MRD CI). Two completely different situations arise on going from Be to heavier alkaline metals. The sp hybridization, which is effective in the Be case, completely disappears when aggregates of Mg and Ca are considered. The potential energy curves relative to Be, Mg and Ca clusters exhibit a very shallow minimum at large distances while a second deeper minimum is characteristic of some Be clusters. This inner minimum is generated by the large interaction of atomic p orbitals at an internuclear separation close to the BeBe distance present in the Be crystal. The bond in Be clusters largely depends on the geometrical arrangement which directly influences the possible sp hybridisation. Very small Be planar structures are always less favored with respect to clusters containing the tetrahedral unit as a part of the whole structure. However, this situation is changed when larger planar clusters are considered, and the highest binding energy per atom (12.0 kcal/mole) from all clusters considered in this work was found for the triplet ground state of the planar Be₇(D_6h) cluster.