  The electronic and structural properties of pure and defective cerium oxide
are investigated using a unified LDA+U approach which allows to treat the
different valence states of Ce occurring for different stoichiometries on a
same ground, without any {\it a priori} assumptions on the defect chemistry.
The method correctly predicts the atomistic and electronic structures of the
\4ce and \3ce bulk phases, as well as the subtle defect chemistry in reduced
materials \xce, which controls the oxygen storage functionality of cerium-based
oxides. The analysis of the energetics highlights the limits of the LDA+U
method and possible extensions are proposed.
