Atom-by-atom and concerted hopping of adatom pairs on an open metal surface

Atom-by-atom and concerted hopping of ad-dimers on the open (100) surface o f fee metals are studied by means of density-functional calculations. The a datom interaction is relatively short ranged, and beyond next-nearest neigh bors ad-dimers are effectively dissociated. Diffusion takes place by a simp le shearing process, favored because it maximizes adatom coordination at th e transition state. This holds for Al, Au, and Rh, and is likely a general result because geometrical arguments dominate over details of the electroni c structure.