Four initial configurations were considered for adsorption of three atoms: three configurations are initially 2D and the fourth is a 3D structure. The three 2D structures are indicated as Line, Offset ( figure ), and Triangle, whereas the 3D structure is labelled as 3D-Triangle. Four initial configurations were also con- For Cu 3 configurations, only 1 three-atom Cu cluster type emerges, that being a triangular structure. For Cu 4 structures however, 4 four-atom Cu cluster types emerge (see figure ). For both Cu 3 and Cu 4 relaxations, clusters of the same type (i.e. same inter-copper arrangements) can differ by both adsorption site and whether or not they have a 2D or 3D morphology. As discussed vide infra, the arrangement of Cu atoms with respect to each other, rather than with respect to the monolayer, is the main factor driving adsorption stability.