[en] The usual procedure using free-atom/ion wave functions to calibrate the Moessbauer isomer shifts is modified in order to include solid state and band structure effects properly. To estimate the electron densities at the nucleus, self-consistent-field Dirac-Fock-Slater wave functions are calculated for several electronic configurations of germanium atoms and ions, taking into account the finite space allowed to these atoms (ions) in solids. Using experimental isomer shift (IS) values, the calibration constants for ⁷³Ge both for the 67.03 and 13.3 keV transitions are calculated, giving the relative change of the nuclear-charge radius ΔR/R = 0.5 x 10^-3 and ΔR/R = 1.2 x 10^-3, respectively. The accuracy of the calibration procedure is discussed. (author)