Small elemental clusters. I. The structures of Be 2 , Be 3 , Be 4 , and Be 5

The geometries and energies of beryllium clusters up to Be 5 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 Be 4 and Be 5 . Furthermore, the effect of d ‐type basis functions on the correlation energy results in an increased binding energy for the clusters. Be 2 is only weakly bound. For Be 3 , the best estimate of the binding energy is 6 kcal/mole for the singlet equilateral triangle. Be 4 is tetrahedral in its ground state and the estimated binding is 56 kcal/mole. The best structure for Be 5 is a singlet trigonal bipyramid, and the binding energy is 88 kcal/mole at the highest level of theory used.