Physics of small metal clusters: Topology, magnetism, and electronic structure

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Abstract
The electronic structure of small clusters of lithium atoms has been calculated using the self-consistent-field, molecular-orbital method. The exchange interaction is treated at the unrestricted Hartree-Fock level whereas the correlation is treated perturbatively up to second order by including pair excitations. This is done in two steps, one involving only the valence electrons and the other including all the electrons. A configuration-interaction calculation has also been done with all possible pair excitations. The equilibrium geometries of both the neutral and ionized clusters have been obtained by starting from random configurations and using the Hellmann-Feynman forces to follow the path of steepest descent to a minimum of the energy surface. The clusters of Li atoms each containing one to five atoms are found to be planar. The equilibrium geometry of a cluster is found to be intimately related to its electronic structure. The preferred spin configuration of a cluster has been found by minimizing the total energy of the cluster with respect to various spin assignments. The planar clusters are found to be less magnetic than expected by Hund’s-rule coupling. For three-dimensional clusters, however, the magnetism is governed by Hund’s rule. The effect of correlation has been found to have decisive influence on the equilibrium topology and magnetism of the clusters. The binding energy per atom, the energy of dissociation, and the ionization potential of the clusters are compared with experiment and with previous calculations. The physical origin of the magic numbers and the effect of the basis functions on the calculated properties have also been investigated.