Calibration of the isomer shift forFe57

The isomer shift calibration constant for ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ is determined by calculating the difference in electronic charge density at the iron nuclei in a ferrous salt and ${\mathrm{K}}_3$Fe${\mathrm{F}}_6$. This is achieved by performing an unrestricted Hartree-Fock calculation for the cluster Fe${\mathrm{F}}_6^{3-}$, and including in the calculation all electrons and all multicenter intergrals. The cluster calculation together with other recent theoretical and experimental data provides the basis for a critical reevaluation and correction of some previous calibration attempts. In all cases but one, agreement with the present work is achieved. The calibration constant is determined to be $(-0.23\mathrm{}\pm 0.02)a_0^3$ mm ${\sec }^{-1\mathrm{}}$.