Interactions between charges and metallic surfaces

We report numerical results for the potential of interaction between two charged particles on a metallic surface. The density functional formalism approach is utilized. The interaction is cast in the form of an image interaction with a correction parameter λ, which depends on the positions of the particles. We find that for molecular distances, λ for the metal mediated interaction, between two charges is approximately given by the average of λ for the self interaction of there charges with the surface. However, the derivatives of the two charges λ, with respect to the particle positions (needed for the evaluation of forces and force constants), cannot be simply expressed as a function of single charge parameters.