All geometries were optimized using the meta-GGA density functional TPSS with a def2-SVP basis set C and H, a def2-TZVP basis set on all other atoms, a Grimme-D3-dispersion correction with Becke-Johnson damping and "tight" con-vergence criteria in OCRA nomenclature. Frequency calculations were used to make sure to find true energetic minima. Energies were calculated with the OPBE GGA functional (ORCA 5.0) or the OLYP GGA functional (ORCA 4.2), the CP(PPP) basis on iron, def2-TZVP basis sets on all other atoms and no dispersion correction. Mössbauer parameters were calculated using the B3LYP hybrid functional, , a Grimme-D3 dispersion correction with Becke-Johnson damping and the same basis sets as the energy calculations. Calibration of the Mössbauer parameter prediction performed by for ORCA 4.2 and an analogous in-house calibration study for ORCA 5.0 resulted in trust regions of ± 0.13 mm s -1 for the isomer shift and ± 0.36 mm s -1 (ORCA 4.2) and ± 0.22 mm s -1 (ORCA 5.0) for the quadrupole splitting. UCSF ChimeraX was used to create molecular graphics.