  The band gap of alkali halides, alkaline-earth oxides, Al2O3 and SiO2
crystals has been calculated using the Perturbed-Ion model supplemented with
some assumptions for the treatment of excited states. The gap is calculated in
several ways: as a difference between one-electron energy eigenvalues and as a
difference between the total energies of appropriate electronic states of the
crystal, both at the HF level and with inclusion of Coulomb correlation
effects. The results compare well with experimental band gap energies and with
other theoretical calculations, suggesting that the picture of bonding and
excitation given by the model can be useful in ionic materials.
