Stability of Small Metallic Clusters : Conceptual and Computational Challenges

Abstract

The flatness of the potential energy surfaces, the possible existence of several local minima, of different spin and space symmetries make especially difficult the theoretical study of atomic clusters. One recalls the Valence-Bond distinction between neutral and ionic states and the various components of binding energies — delocalisation and charge and spin correlation within the valence band — hybridization, dispersion and dynamical correlation effects going out of the valence band. The extent and qualitative effects of these various components is first illustrated on filled bands and nearly filled band problems (neutral and positively charged clusters of alkaline earth and rare gas atoms) and on half-filled band situations (alkali and transition metal clusters). The presently proposed lithium cluster conformations correspond to ionic states, but possible neutral structures with ordered spin waves should be searched. The intra-atomic ferromagnetism of the transition metals introduces a constraint neglected by ab initio RHF approaches and taken into account in unrestricted HF and LSD formalisms.