Electron Localization at Metal Surfaces
  We investigate some surfaces of a paradigmatic sp bonded metal--namely, Al(110), Al(100), and Al(111)--by means of the "electron localization function" (ELF), implemented in a first-principle pseudopotential framework. ELF is a ground-state property which discriminates in a very sharp, quantitative, way between different kinds of bonding. ELF shows that in the bulk of Al the electron distribution is essentially jelliumlike, while what happens at the surface strongly depends on packing. At the least packed surface, Al(110), ELF indicates a free-atom nature of the electron distribution in the outer region. The most packed surface, Al(111), is instead at the opposite end, and can be regarded as a jellium surface weakly perturbed by the presence of the ionic cores. 
L. De Santis and R. Resta