Calibration of theFe57Isomer Shift

The $s$-electron density at the iron nucleus in KFe${\mathrm{F}}_3$ is calculated as a function of the Fe-F distance, using the Heitler-London model and the Hartree-Fock ionic wave functions. By comparing the calculated "overlap-induced" $s$-electron density with the hydrostatic pressure isomer shift measurements in KFe${\mathrm{F}}_3$, the relative change of the ${\mathrm{Fe}}^{57}$ nuclear radius is found to be -5.2×${10}^{-4\mathrm{}}$, which is to be compared with the value -1.8×${10}^{-3\mathrm{}}$ derived previously by Walker et al.