Nuclei-specific spectroscopy methods, such as X-ray absorption spectroscopy or Mössbauer spectroscopy are thus required for structural elucidation. Because of the detailed information on iron oxidation and spin states offered by Mössbauer spectroscopy, it has been used especially extensively to study FeNC catalysts. A Mössbauer doublet signal is characterized by the isomer shift and the quadrupole splitting. The isomer shift depends linearly on the s-electron density at the nucleus, while the quadrupole splitting is defined by the electric field gradient, making it highly sensitive to the symmetry of the electron distribution around the nucleus. Both Mössbauer parameters show a temperature dependence, making low temperature experiments on FeNC catalysts highly relevant for comparisons with computational data that are formally carried out at 0 K. The currently dominant perspective on active sites in FeNC catalysts is that iron atoms are surrounded by a fourfold nitrogen coordination sphere that are embedded in a carbon matrix, which is assumed to be graphene-like . However, questions regarding the type of N-donor, the presence and nature of axial ligands, the oxidation state (Fe(II) or Fe(III)) and the spin state of iron are still being debated. Recent in situ -measuring the catalyst in an electrochemical cell with an applied potentialand operando -experiments under applied potential and oxygen saturation -Mössbauer studies on FeNC catalysts shed light on the different species appearing during the ORR oxidation cycle. The in situ conditions showed spectroscopic signatures of iron high spin states which have been attributed to the active site. The interpretation of experimental spectroscopy data is often supported by quantum chemical studies of model systems. . Both molecular and periodic calculations can be used for structural elucidation of FeNC catalysts, where both approaches have strengths and weaknesses. Molecular models are structurally flexible and allow for direct comparisons with experimentally available porphyrin and phthalocyanine complexes for which Mössbauer parameters and other spectroscopic properties have been studied extensively. Due to the associated computational cost, the π system that can be considered in molecular models is limited. It is therefore unclear whether the graphene-like structure assumed in FeNC catalysts is represented appropriately. Periodic models incorporated with atomic and/or plane-wave basis sets are suitable for large crystalline systems and detailed analyses of electronic and magnetic structures. Here, too, the computational cost associated with a larger system size limits the extent of the graphene-like layer that the active site is embedded in, which may lead to artificially interactions between iron atoms in adjacent unit cells. For both model types, it is thus important to identify a sufficient model size, which has not been systematically evaluated to date.