The nature of the ground state of the V centre in ionic oxides is discussed, where two holes
are trapped by a cation vacancy. Experiment appears to indicate an S= 1 ground state for
BeO and Al 2 O 3 and possibly ZnO, with S= 0 in MgO and CaO. Simple theoretical
arguments suggest S= 0. It is shown that a weak mixture of (O 0 O 2-) configurations into the
dominant (OO-) configuration can explain the observations. Quantitative calculations are
given for MgO, and suggest the singlet state is lowest by about 30 cm-1.

The nature of the ground state of the V centre in ionic oxides is discussed, where two holes
are trapped by a cation vacancy. Experiment appears to indicate an S= 1 ground state for
BeO and Al< sub> 2 O< sub> 3 and possibly ZnO, with S= 0 in MgO and CaO. Simple
theoretical arguments suggest S= 0. It is shown that a weak mixture of (O< sup> 0 O< sup> 2-
) configurations into the dominant (O< sup>- O< sup>-) configuration can explain the
observations. Quantitative calculations are given for MgO, and suggest the singlet state is …