Electron correlation in small metal clusters. Application of a theory of self-consistent electron pairs to the Be4 system

A knowledge of the properties of small metal particles is essential to the understanding of catalysis on a molecular level. In this regard, one particularly important property is the rate at which the dissociation energy of a small metal cluster approaches the bulk cohesive energy. The present research concerns the effect of electron correlation on the dissociation energy of a particularly stable beryllium cluster, the tetrahedral Be4 system. A contracted Gaussian basis set of size Be(9s 4p/5s 2p) was adopted in conjunction with the recently developed theory of self-consistent electron pairs (SCEP). Several new theoretical and computational wrinkles are discussed, including the incorporation of the SCEP/coupled electron pair approximation (SCEP/CEPA). The Be4 results provide strong evidence for the reliability of the Hartree–Fock approximation for alkaline earth cohesive energies. As suggested earlier the Be4 dissociation energy appears to be ∼40 kcal/mole. Analogous studies of the Be2 molecule are reported.