  The electronic structure and optical absorption spectrum of V^{0} and V^{-}
center in MgO are investigated using first-principles calculations based on
density functional theory. It is demonstrated that the configuration with
distortion is energetically favorable in PBE0 functional. The six O atoms
around Mg vacancy are non-equivalent due to the distortion. The defect states
localized in the band gap are obtained by applying hybrid functional
calculations, which is very important for optical properties. At ambient
condition, the absorption peak at ~2.5 and ~2.6 eV is assigned to V^{0} and
V^{-} center, respectively. With increasing pressure, the absorption peak shows
blue shift. The trend is consistent with the experimental observation.
