The interaction between an atom and a surface at large separation

A simple expression is generated for the coefficient C₃ of the asymptotic dispersion interaction ~z^?3 between an atom and a surface. The basis of the derivation is the assumption of simple forms for the frequency dependence of the polarizability of the atom and the dielectric function of the solid. The expression yields the known value of C₃ within a few percent for all cases for which it has been calculated previously. We use it to generate C₃ values for a large number of systems not previously treated.