Small beryllium clusters: Be2, Be4, and Be5

With the exception of the dimer, the small clusters of beryllium have yet to be successfully characterized by spectroscopy. In this study, the geometries and dissociation energies of Be4 and Be5 are computed using highly accurate coupled-cluster methods. Dunning's correlation consistent polarized-valence basis sets are employed, and the energies computed for the tetramer are extrapolated to the complete basis set (CBS) limit. The importance of triple and quadruple excitations from the zeroth-order wavefunction are assessed for both clusters. Triples are found to be essential to describing the clusters, but quadruples are found to be of only minor importance. Computations on the challenging Be2 molecule are reported for the purpose of benchmarking our results. For the tetramer, fundamental vibrational frequencies and rotational constants are obtained by applying second-order vibrational perturbation theory (VPT2) to a full quartic force field computed at the c~CCSDT(Q)/CBS level of theory.