Interionic potentials for alkali halide crystals derived by the hybrid Thomas-Fermi-Dirac method

The first results of a systematic study of interatomic potentials in sixteen alkali halides derived using the hybrid Thomas-Fermi-Dirac method are presented. Both the basic method and modifications to (a) correct the exchange energy to exclude the self-exchange part and (b) scale the kinetic and exchange energy terms are employed. The interactions between ion pairs are calculated at a set of interionic distances in a range corresponding to ± 30% deviations from the appropriate measured equilibrium separation. In this way a tabular function for the lattice energy/ion pair is constructed and predicted values of the equilibrium nearest neighbour interionic distance, the equilibrium lattice energy/ion pair and the Smith stiffness parameter are found numerically. The derived short-range interactions between ion pairs are fitted over the whole range to the analytic form A exp($\text{?R}\text{ρ}$) + CR^?6. It is shown that the use of an expression for the lattice energy/ion pair involving these analytic forms introduces significant errors in the predicted physical parameters.