Use of the unrestricted-Hartree-Fock method in the study of clusters of metal atoms

Some time ago Marshall, Blint, and Kunz [Phys. Rev. B 13, 333 (1976)] critically compared various simple theoretical techniques—such as the restricted- and unrestricted-Hartree-Fock methods (RHF and UHF) as well as the generalized valence bond (GVB) and scattered wave (SCF $X\alpha$) approaches—applied to the study of clusters of lithium atoms. They concluded that the UHF is particularly attractive considering a balance of reliability and computational cost. Here we redo the discussion adding some new results for Li clusters both at the UHF and the more precise ab initio configuration-interaction (CI) levels for comparison. We want to emphasize not only the total binding energies as in the paper by Marshall et al. but also their decomposition into additive and nonadditive contributions and into a multibody expansion. The latter is quite relevant to the problem of using such calculations of finite clusters of metal atoms to attempt predictions of some properties of the bulk metal (e.g., its cohesive energies and adsorption capacities).