Covalency versus Overlap Distortion in the Mössbauer Isomer Shift

Charge-density differences $\Delta \rho$ opserved in Mössbauer isomer-shift (IS) experiments on metal atoms embedded in rare-gas matrices are studied in perturbed-atom and molecular-orbital models. Calculations performed for ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ in an argon matrix and a full variational treatment of Fe${\mathrm{Ar}}_{12}$ clusters show that overlap distortion and covalent contributions to $\Delta \rho$ compete and nearly cancel. This result is seen to be general, and implications for free-atom models and IS pressure dependence are discussed.