Surface friction constant and range of dynamical interaction between adatoms on metal surfaces

The electronic contribution to the surface friction constant of an adatom on a metal surface is calculated using: (i) the Dirac density matrix for an infinite barrier model of an inhomogeneous electron gas, and (ii) two simple models of screened potentials at the surface based on reliable asymptotes in and outside the surface respectively. Though the results are clearly approximate, attention is drawn especially to the predictions of the models concerning long-range dynamic interactions between adatoms on metal surfaces.