Covalency versus overlap distortion in the mössbauer isomer shift

Charge-density differences Δρ 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 Fe57 in an argon matrix and a full variational treatment of FeAr12 clusters show that overlap distortion and covalent contributions to Δρ compete and nearly cancel. This result is seen to be general, and implications for free-atom models and IS pressure dependence are discussed.