ON THE CALCULATION OF THE LATTICE ENERGY OF IONIC-CRYSTALS USING THE DETAILED ELECTRON-DENSITY DISTRIBUTION .1. TREATMENT OF SPHERICAL ATOMIC DISTRIBUTIONS AND APPLICATION TO NAF

Ewald's method of accelerated convergence [Ewald (1921). Ann. Phys. (L eipzig), 64, 253-287] is generalized to calculate the electrostatic po tential of a crystal in which the atoms have overlapping spherical den sities, The algorithm is applied to the cubic NaF crystal. The potenti als at the Na and F nuclei are calculated for the free-ion model and f or the results from a kappa refinement of the experimental data of How ard and Jones [Acta Cryst, (1977), A33, 776-783]. The kappa refinement indicates an incomplete charge transfer but gives an electrostatic en ergy close to that of the point-charge model with full charge transfer and a lattice energy that is in good agreement with the experimental value.