As previously mentioned, 4 atom Cu clusters show a greater preference for 3D growth than 3 atom clusters. Unlike Cu 3 , where there is only 1 distinct copper structure, that being a triangular structure (both 2D and 3D), there are 4 unique 4 atom Cu clusters (see figure ). Similarly to 3 atom clusters, the most stable of these structures are the rhombic structures that resemble the D 2h ground state configuration of a neutral four atom Cu cluster in gas phase . Both 2D and 3D structures in which copper atoms arrange in this manner have the largest metal-metal interaction energies of all copper adsorption structures. It is also interesting to note that all adsorption modes with this structure are adsorbed at the chalcogen site.