We present DFT-GGA periodic calculations to study the formation of F 21 vacancy on MgO(100) surface. On stoichiometric bare surface, the cost of the defect formation is prohibitive even if it results from migration of surface oxygen atom on a terrace (Schottky process at the surface). The corresponding ground state is high spin and only one electron instead of two is trapped in the cavity leading to the formation of F-S(+) center. On the contrary, the presence of two OH-groups adsorbed near by allows the formation of F-S(2+)(2OH(-)). This process starting from hydrated perfect surface only costs 4.20 eV. (C) 2004 Elsevier B.V. All rights reserved.