  We revisit the well-known Mollwo-Ivey relation that describes the "universal"
dependence of the absorption energies of F-type color centers on the lattice
constant $a$ of the alkali-halide crystals, $E_{\mbox{abs}}\propto a^{-n}.$ We
perform both state-of-the-art ab-initio Quantum Chemistry and post-DFT
calculations of F-center absorption spectra. By "tuning" independently the
lattice constant and the atomic species we show that the scaling of the lattice
constant alone (keeping the elements fixed) would yield $n=2$ in agreement with
the "particle-in-the-box" model. Keeping the lattice constant fixed and
changing the atomic species enables us to quantify the ion-size effects which
are shown to be responsible for the exponent $n \approx 1.8$.
