Five density functionals including GGA (generalized gradient approximation) (BP86), meta-GGA (TPSS), hybrid meta-GGA (TPSSh), hybrid (B3LYP), and double-hybrid functionals (B2PLYP) were calibrated for the prediction of Fe-57 Mossbauer isomer shifts on a set of 20 iron-containing molecules. The influence of scalar relativistic effects and the basis set dependence of the predictions were investigated.