We have performed ab initio Hartree-Fock cluster model calculations on the electronic structure and properties of neutral and charged oxygen vacancies, the F color centers, at various sites of the MgO(001) surface. Sub-surface, surface, step, and corner sites have been considered. For each site we have determined the optimal structure, the relative stability of neutral versus charged vacancies, the formation energy, and, for the paramagnetic F+ centers, the spin distribution as given by the isotropic hyperfine coupling constants of the unpaired electron with the surrounding Mg2+ nuclear spin. The barrier for diffusion of oxygen ions in the presence of F2+ centers has been estimated for the migration from the sub-surface to the surface, from a terrace site to another terrace site, and from a step to a terrace site. (C) 1998 Elsevier Science B.V. All rights reserved.