Theoretical investigations of the structures and binding energies of Be_n and Mg_n (n=3-5) clusters

We have determined the equilibrium geometries and binding energies of Be and Mg trimers, tetramers and pentamers using single and double excitation coupled cluster (CCSD) and complete active space self-consistent-field (CASSCF) multireference configuration interaction (MRCI) wave functions in conjunction with extended atomic basis sets. Our best estimates of the cluster binding energies are 24, 83, and 110 kcal/mol for Be₃, Be₄, and Be₅; and 9, 31, and 41 kcal/mol for Mg₃, Mg₄, Mg₅, respectively. A comparison of the MRCI and CCSD results shows that even the best single-reference approach (limited to single and double excitations) is not capable of quantitative accuracy in determining the binding energies of Be and Mg clusters.