Inhomogeneous contraction of interatomic distances in metallic clusters: Calculations forCsnandOCsn

The equilibrium geometrical structures of ${\mathrm{Cs}}_{\mathit{n}}$ and ${\mathrm{OCs}}_{\mathit{n}}$ clusters have been obtained by a method that, in the framework of density-functional theory, describes the ion-electron interaction by means of a pseudopotential that is spherically averaged about the cluster center. In the size range studied (up to n=78 and 70, respectively) the clusters present well-separated atomic layers for which the distances to the two closest neighbors of each atom have been analyzed, showing that there is an inhomogeneous shrinking of the geometrical structure, in the sense that the distances obtained for atoms in the inner layers are lower than those for the outer ones. The analysis of the growing of the clusters suggests that the equilibrium geometries are those for which any atom of the aggregate has its closest neighbor at a distance lower than the nearest-neighbor distance of the pure bulk metal. For small (n≤20) pure clusters, geometries of high symmetry have been found, which are completely modified by the presence of the oxygen impurity. A regular octahedron is formed around the oxygen atom in most cases.