The structure of NiN and PdN clusters: 4≤N≤23

Abstract

Stable geometrical structures of Ni_N and Pd_N clusters (N=4–23) are identified using a corrected effective medium (CEM) theory. Structural optimization is accomplished by simulated annealing using analytic derivatives to determine the interatomic forces. Unique structural features of these metal clusters are noted, especially in relation to the bulk and surface phases of these metals and to structures commonly associated with rare gas clusters. Elucidation of the general features of cluster growth leads to the principle that transition metal clusters generally maximize the minimum coordination of any atom. By contrast, rare gas clusters maximize the number of interatomic distances close to the optimal distance for the pairwise interaction between rare gas atoms. The latter can be interpreted as the packing of hard balls. Structural transformations between isomers of similar energy are also examined for selected sizes.