Breakdown of atomic superposition for the helium scattering potential of oxygen on nickel (001)

Self-consistent calculations of the charge density of the c(2×2) phase of oxygen on nickel (001) (i) give a helium scattering potential which in excellent agreement with the shape, height and width of that obtained by Rieder by fitting his atom-surface scattering data: (ii) justify th use of a hard-wall fit because of a 4.2 Å−1 averrage softness parameter; and (iii) rule out superposition of atomic or ionic charge densities for this and similar systems.