Predicted Mössbauer Parameters
From previous studies it is clear that hybrid functionals are best suited for the prediction of Mössbauer parameters. GGA and meta-GGA functionals perform less favourably, and double-hybrid functionals do not show any significant improvement that would justify the higher computational cost.19 Therefore, three hybrid density functionals, TPSSh (10% HF exchange), B3LYP (20% HF exchange) and PBE0 (25% HF exchange), in combination with the CP(PPP) basis set for iron and the def2-TZVP basis set for all other elements were selected for this calibration study (see Computational Details). We reiterate that the main purpose of this study is to define uncertainty estimates in isomer shift and quadrupole splitting predictions specifically for FeN4-environments.