For purposes of comparing models of surface atomic structure with atom diffraction data, it is proposed that the surface of critical charge density for atom scattering be obtained from the superposition of atomic charge densities. The results of this method, which has the advantage of permitting fast calculation for many structures, including complex ones, are compared with the self-consistent calculations of Hamann for GaAs(110) and Ni(110). The method is then applied to the Si(111) surface and used to illustrate the effects of atom shifts on corrugations of the surface of critical charge density.