Accurate ⁵⁷Fe Mössbauer Parameters from General Gaussian Basis Sets

The prediction of isomer shifts in ⁵⁷Fe Mossbauer spectra is typically achieved by building calibration lines using the values of the density at the nuclear position. Using Slater-type orbital basis or large and specific Gaussian-type orbital basis has been thus far mandatory to achieve accurate predictions with density functional theory methods. In this work, we show that replacing the value of the density at the nucleus by the density integrated in a sphere of radius 0.06 au centered on the Fe nuclei yields excellent calibration lines (r² = 0.976) with a high predictive power (q² = 0.975, MAE = 0.055 mm·s^-1) while using the conventional def2-TZVP basis set and X-ray geometrical parameters. Our data set comprises 69 ⁵⁷Fe-containing compounds and 103 signals. We also find B3LYP performing significantly better than the PW91 functional.