ALKALI-METAL CLUSTERS - AN S-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 Ḧckel 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 (MOs) of the p band stabilizing the highest occupied MOs of the s band. © 1990 The American Physical Society.