Metal clusters are a strange lot.
Here the electrons are free to move all around the cluster. Infact, they are so free that they forget which atom they belonged to. To them all atoms look the same and appear to form a uniform positive background. The theoretical understanding of metal clusters has most often relied on the Jellium model of clusters. The jellium model assumes that the valence electrons of all the atoms in a cluster are free and see a uniform positively charged jellium formed by the ions.