Small elemental clusters. I. The structures of Be2, Be3, Be4, and Be5

The geometries and energies of beryllium clusters up to Be5 are examined using ab initio molecular orbital theory. Allowances are made for electron correlation with Møller—Plesset perturbation theory to fourth order. Correlation is found to have a dramatic effect on the relative energies of the several structures examined for Be4 and Be5. Furthermore, the effect of d‐type basis functions on the correlation energy results in an increased binding energy for the clusters. Be2 is only weakly bound. For Be3, the best estimate of the binding energy is 6 kcal/mole for the singlet equilateral triangle. Be4 is tetrahedral in its ground state and the estimated binding is 56 kcal/mole. The best structure for Be5 is a singlet trigonal bipyramid, and the binding energy is 88 kcal/mole at the highest level of theory used.