A comment on the surface dielectric sum rule

Abstract We show that our ⨍;-sum rule for the electron response function at a surface is not satisfied in the ϵ-approximation (semi-classical infinite barrier model), and the recent objection to our procedure by Griffin and Harris is shown to be invalid. Although this approximation is useful for many purposes it does not give a correct result for the plasmon contribution to the surface energy, a fact which is directly connected with its failure to satisfy the ⨍;-sum rule.