Equilibrium geometries and electronic structures of small sodium clusters

We report self-consistent local-spin-density calculations which provide, for the first time, the equilibrium geometries of the sodium clusters Nan and Nan+ with n<~8 and n=13, without making any a priori assumptions. Our results are in excellent agreement with recently obtained photoionization appearance potentials and electron-spin-resonance spectra. We find rapid formation of the metallic bond with a dominance of closely packed structures in two and three dimensions, and propose a simple model to account for it.