In Figure , the Mössbauer parameters of the most stable electronic configuration of all evaluated models are compared to experimental Mössbauer doublets labeled D1/D1a, D1b, D2, and D4 in the literature measured at low temperatures under different conditions. We highlight that this is to the best of our knowledge the most complete literature survey of Mössbauer parameters for FeNC systems, see also Table and Figure . Experimental Mössbauer spectra are interpreted by fitting a number of doublets (and, if necessary, singlets and sextets) corresponding to the number of iron sites The data points are labeled with the corresponding model name and axial ligands where applicable. The overlapping color-coded ovals depict experimental low temperature (T < 77K) Mössbauer doublets from the literature obtained under as-prepared, in situ, operando, and ex situ conditions. The shapes of the ovals are constructed from the trust regions of the calculated Mössbauer parameters as defined by previous calibration studies (isomer shift: ± 0.13 mm s -1 , quadrupole splitting ± 0.22 mm s -1 ). The color code is as follows: doublets D1/D1a (dark green) represent the commonly most intense signal appearing at all measurement conditions; D1b (light green) is less commonly used for fitting and can only be observed for inactivated catalysts; D2 (blue) also appears under all conditions; D4 (cyan) only appears at operando conditions. The assignment strictly follows the respective papers. More information can be found in Figure and Table .