Abstract Based on a semicontinuum model, the electrostatic, the polarisation, and the repulsive energy change of a lattice is calculated numerically, as a function of the relaxation of the nearest neighbouring ions of a substitutional impurity in an alkali halide crystal. It is found that for a particular displacement, the total energy change of the lattice is a minimum. Thus the heat of formation of a dilute solid solution is obtained. Here, we report calculations on the heat of formation of the following systems-Na + in LiCl, Li + in NaCl, K + in NaCl, Na + in KCl, Rb + in NaCl, Na + in RbCl, F − in NaCl, Cl − in NaF. Br − in NaCl and Cl − in NaBr.