Crystal potentials have been derived for SrCl2and Na2S by first calculating the pair-interactions of the ions in the electron-gas approximation and then refitting the most crucial parameters to crystal properties. For both these materials it seems to be impossible to obtain a perfect description of crystal properties using current two-body interactions, so various compromise potentials have been studied that emphasize the fit to particular properties at the expense of others. Defect calculations with these potentials lead to some conclusions about the effect of weighting the fitting of certain properties (elastic constants, dielectric constants, phonon frequencies) but this is a field not much explored hitherto and so it is not known how general these conclusions might be. All the potentials confirm that the primary defects are anion Frenkel defects in SrCl2and cation Frenkel defects in Na2S. Calculated activation energies for defect migration appear to be on the low side, except for S2-migration in Na2S for which the experimental Arrhenius energy is unusually small. Some properties of defects in NaCl:S2-are discussed in the Appendix.