The theory is given for the electronic structure of the F⁺_s surface centre in MgO. This defect consists of an electron trapped at an anion vacancy on a [001] surface. The calculations use molecular Hartree–Fock methods, treating a small number of ions in detail and representing the others by appropriate point ions. The basis functions include both ionic functions and functions centred within the vacancy on its axis. An approximate treatment of lattice distortion and polarisation is also included. The results of this “cluster” approach give spin-resonance parameters and the energies and oscillator strengths of optical transitions.

Predicted values of the isotropic hyperfine interaction are in reasonable agreement with experiment. The optical transitions do not seem to correspond to those detected in reflectance measurements; other possibilities are discussed.