Systematicab initioconfiguration-interaction study of alkali-metal clusters: Relation between electronic structure and geometry of small Li clusters

The electronic and geometric structure of small neutral Liₙ (n=2--9) and cationic Liₙ{}⁺$ (n=2--9) clusters is investigated with the optimal self-consistent-field energy and geometry search and with the multireference diexcited configuration-interaction method. The optimal geometries of neutral and cationic Li clusters are very different. Very small Liₙ (n{≤}6) clusters prefer planar geometries [deformed sections of the (111) fcc-lattice plane]. The optimal geometries of somewhat larger Liₙ (6{≤}n{≤}9) clusters are composed from condensed deformed tetrahedra. This switch in the geometrical structure of clusters can be easily understood on the basis of some simple qualitative arguments. Predicted relatively large stabilities of Li₄ and Li₈ as well as of Li₃{}⁺$ and ${Li}₉⁺ can help to interpret the ``magic numbers'' observed when alkali-metal clusters are prepared and detected under differing experimental conditions. The theory predicts stability of relatively small doubly charged alkali-metal clusters which seems to be in variance with the customary ideas of ``Coulomb explosion.'' The general trend of the dependence of ionization potentials of alkali-metal clusters on the cluster nuclearity agrees with experiments. The present work confirms some conclusions of the ``electron-shell model'' but puts them on a more general basis, independent of conceptual and methodological details. Furthermore, it reveals new aspects and leads to predictions.