Unexpected relaxation of a Ag layer on Cu(111)

The structure and energetics of a layer of Ag deposited on Cu(111) has been computed using the embedded atom method (EAM). The lowest energy structure of the overlayer is a pseudoepitaxial arrangement of the Ag with 8 unit cells of Ag aligned with 9 unit cells of Cu. The structure is consistent with experimental observations and similar theoretical calculations by Mottet, Tréglia and Legrand. The unexpected aspect of the theoretical results is the calculated atomic relaxations of the surface. The surface is found to have the lowest vertical position where the Ag atoms are in top sites over the Cu. This disagrees with the usual expectations based on hard sphere ideas. This result is explained in terms of the improved bonding between the substrate and an adsorbate in a top site when the substrate forms a local depression around the site.