Electronic states and geometries of small Be clusters

Abstract

A b initio calculations were carried out on all regular geometric configurations of beryllium clusters containing three through seven atoms. Restricted Hartree–Fock, unrestricted Hartree–Fock, and fourth-order Mo/ller–Plesset perturbation theory (MP4) using a 6-31G basis set were used to study singlet, triplet, and quintet spin states of each cluster. The septet state of the most stable nuclear configuration of Be7 was also examined using unrestricted Hartree–Fock theory. In addition, Be4 was examined as a prototype for larger clusters, for which accurate calculations may not be computationally feasible. The most stable symmetric nuclear configuration and electronic state of Be4 was determined at increasingly improved levels of approximation. Hartree–Fock, configuration interaction, and Mo/ller–Plesset perturbation theory calculations were carried out using 6-31G, 6-311G, 6-31G*, and 6-311G* basis sets. Binding energies for the most stable configuration of each cluster were improved by MP4 calculations in a 6-31G* basis set. Be–Be internuclear separations for the lowest electronic state of each cluster range from 1.9 to 2.5 Å. MP4 calculations predict singlet ground states for Be3, Be4, and Be5, whereas Be6 and Be7 are found to have quintet ground states.