A study of the variational convergence of theFcentre energy levels of LiF

SCFMO calculations have been made on the F centre states of LiF using a crystal fragment whose potential closely approximates that of the full crystal. The effect of including electrons of up to third neighbours has been studied. The most extensive calculation gives a binding energy for the 2 S state in the unrelaxed lattice of 5·25 eV and a 2 S→2 P transition energy of 4·6 eV. Lattice relaxation has been calculated in a model appropriate to all lithium halides in which only nearest neighbour electrons are considered. For LiF an outward displacement of nearest neighbour ions by 2·5 per cent of the lattice parameter has been calculated. For the other halides the predicted displacements are smaller.