Unrestricted-Hartree-Fock approach to cluster calculations: Application to lithium

In this paper, we examine theoretical techniques for studying clusters of atoms. We examine the traditional restricted-Hartree-Fock method, the general-valence-bond method, and the space-spin unrestricted-Hartree-Fock method. We perform a series of calculations to determine the structure of fine particles of lithium and conclude that the most accurate available simple Hartree-Fock-type method is the unrestricted-Hartree-Fock method. These results are contrasted to recent studies on similar systems employing the self-consistent field Xα scattered-wave approach, and we conclude on the basis of inferences from experimental data that the unrestricted-Hartree-Fock method produces results in fair agreement with the Xα method for such metal systems, and that important differences exist between both methods.