Alkali-metal clusters: Ans-band uncorrelated versus (s+p) highly correlated problem

In solid-state physics, alkali metals are usually considered as s-band-weakly-correlated systems and this view was recently supported by the qualitative success of a H\"uckel topological description in the prediction of the structures of small alkali-metal clusters. Paradoxically ab initio calculations show the dramatic role played by p atomic orbitals and electronic correlation. An ab initio diabatic analysis of the diatom establishes that the p atomic orbitals are essentially involved through the angular electronic correlation of the ionic valence-bond structures and a lowering of the energy of these forms. From accurate calculations on Li₂, Li₂⁺, and Li₂^-, nonempirical s-band interactions are determined and the corresponding two-body Hamiltonian appears to predict correctly both energies and structures of small Liₙ, Liₙ⁺, and Liₙ^- clusters. Precise comparisons with ab initio results show, however, the influence of collective effects in the p band, the interaction with the low-lying molecular orbitals (MO's) of the p band stabilizing the highest occupied MO's of the s band.