Calibration of 57Fe isomer shift from ab initio calculations: can theory and experiment reach an agreement?

Using linear response approach to the Mössbauer isomer shift, the calibration constant alpha((57)Fe) was obtained from high level ab initio calculations carried out for a representative set of iron compounds. The importance of the effects of relativity and electron correlation for an accurate description of the (57)Fe isomer shift is demonstrated on the basis of the Hartree-Fock, coupled cluster with singles and doubles and of the double hybrid density functional calculations. A reliable value of the calibration constant (alpha((57)Fe) = -0.306 +/- 0.009 mm s(-1)) was obtained with the use of the B2-PLYP double hybrid density functional. This value is in good agreement with the experimentally estimated constant of -0.31 +/- 0.04 a(0)(3) mm s(-1) and can be recommended for theoretical modeling of (57)Fe isomer shifts.