The motion of isolated adatoms and small clusters on a crystal surface was investigated by a novel and efficient simulation technique. The trajectory of each atom was calculated by molecular dynamics, but the exchange of kinetic energy with the crystal lattice was included through interactions with a "ghost" atom. This atom represented surface atoms of the lattice and was subjected to random and dissipative forces that were related by the fluctuation-dissipation theorem. The diffusion process was characterized by measurements of the velocity autocorrelation function, mean square displacement, directional correlations between hops, and the mean displacement per hop. In addition, the rate of evaporation of single adatoms and the rate of dissociation of clusters were considered. The diffusion of an isolated adatom was found to be somewhat faster than that predicted by the classical rate theory for an activated process. This effect was a result of diffusion jumps of several atomic diameters that occurred preferentially at high temperatures. But Arrhenius behavior was observed over the entire range of temperatures below the melting point. Dimers and larger clusters were found to diffuse more slowly than individual atoms, but with a smaller apparent activation energy. These results did not exhibit the high-temperature anomalies that were inferred from some experimental data on surface mass transport. In a subsequent paper the method was extended to treat mass transport in the layer of adatoms and clusters that results from a dynamic equilibrium with the vapor phase.

Molecular Dynamics of Surface Diffusion. I. the Motion of Adatoms and Clusters. Tully, J.C., Gilmer, G.H., Shugard, M.: The Journal of Chemical Physics, 1979, 71[4], 1630-42