We report the first general calculation of the empirical parameters appearing in the Mollwo-Ivey relation between absorption energy and interionic spacing for defects in ionic solids. The calculation is based on the Vinti sum-rule formulation of the Mollwo-Ivey law [Phys. Rev. Lett.69, 184 (1992)], a self-consistent Hartree-Fock-Slater procedure for defect ions, and a host-crystal environment modeled in the point-ion approximation. The Ivey exponent is reproduced in most cases to within better than 20% using this model in the alkali halides. The wide range of Ivey exponents can be qualitatively understood in terms of the compressibility of the defect.