A b initio spin–orbit full configuration-interaction calculations in the context of relativistic effective core potentials are reported for the weakly bound metal dimer LiBe, a three-valence-electron system. The effects of basis set on the energies of valence and Rydberg states of the cluster are discussed, as are the effects of configuration space selection on the energy of the latter states. Results at the dissociative limit are compared to the experimental atomic spectra. Potential-energy curves and spectroscopic constants are presented for the ground state and fourteen excited states, which includes the Li and Be 2p valence states, the Li 3s, 3p, 3d, and 4s Rydberg states, as well as three low-lying states of the molecular cation.
Ab 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 Mlhller-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 Be 7 was also examined using unrestricted Hartree-Fock theory. In addition, Be 4 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 Be 4 was determined at increasingly improved levels of approximation. Hartree-Fock, configuration interaction, and Mlhller-Plesset perturbation theory calculations were carried out using 6-31 G, 6-311 G, 6-31 G*, and 6-311 G* 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 A. MP4 calculations predict singlet ground states for Be 3 , Be 4 , and Be s , whereas Be6 and Be 7 are found to have quintet ground states. Trigonal Bipyramid Pentagon Linear FIG. 1. Five-atom clusters.
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