  When two or more atoms bind to a solid surface, the substrate can mediate an
interaction between them. In this paper, we use density-functional theory to
quantify the substrate-mediated pair interaction between two adatoms on a
compressively strained Ag(111) surface and on unstrained Ag(111). On the
strained surface, the elastic interaction is significant over the short range
and leads to a net attraction between two adatoms. However, at the longest
distances probed, the interaction is primarily electronic and repulsive. The
repulsion can be as strong at 50 meV, and it forms a ring-like structure around
an atom. On unstrained Ag(111), the interaction is primarily electronic in
origin and it is weak relative to the interaction found on the strained
surface. We calculate the energy barrier for an isolated atom to diffuse in
each of these systems. For the strained surface, the magnitude of the diffusion
barrier is comparable to that of the adsorbate interaction. We discuss the
implications of our findings for growth at surfaces.
