The optical transition energies of a point defect in MgO(100), consisting of an oxygen vacancy with one trapped electron and an OH-group adsorbed nearby the surface oxygen vacancy (the F-s(+)(OH-) center), have been studied by means of ab initio multiconfiguration second order perturbation theory (CASPT2) and Density Functional (DFT) calculations on embedded cluster models. We find that a method that takes explicitly into account electronic correlation effects is required to properly predict the first transition energies. The lowest transition energy of the F-s(+)(OH-) center on a terrace site is very similar to the one reported in previous studies of F+ centers on a regular MgO(100) surface (F-s(+) center). Experiments have difficulty distinguishing between these two kinds of point defects based solely on the optical spectra. We have also considered the situation where the vacancy is located at low-coordinated sites like steps and corners. The results show that the loss of coordination decreases the transition energies in agreement with what has been reported in previous studies for "classical" F-s(+) centers. (c) 2006 Elsevier B.V. All rights reserved.