ON THE CONVERGENCE OF MANY-BODY EXPANSION OF THE POTENTIAL-ENERGY FORALKALI-METAL CLUSTERS

The convergence properties of the many-body expansion of the potential energy function for homonuclear alkali metal clusters is examined on the basis of a direct evaluation of many-body contributions, carried o ut by means of ab initio correlated quantum chemical methods. The stud y of a series of Li-N clusters (N = 2,...8) shows that the expansion d oes not possess any evident convergence, at least up to the highest nu clearity considered. The energy terms for N- and (N + 1)-body interact ions have opposite sign and are characterised by increasing magnitude for increasing nuclearity. Using ab-initio energy data, a simplified a nalytical potential is derived, which includes two; and three-body ter ms only. The reliability of the truncated potential in predicting opti mal shapes and stabilities of Li-N clusters is discussed.