The expected size of the structural models of several dozen atoms limits the electronic structure methods that can be applied. Wavefunction methods cannot be used due to their high computational cost even though iron is notorious for its often multireference electronic structures due to partially occupied d-orbitals and many accessible electronic states. Instead, FeNC catalyst models are usually studied with density functional theory. Nevertheless, the electronic complexity of iron warrants a comprehensive view of all probable electronic structures and spin states in FeNC models. In this work, we investigate two typical FeNC active site models with density functional theory: the pyridinic-type FeN 4 C 10 (model A) and the pyrrolic-type FeN 4 C 12 (model B), see Figure . We employ both molecular and periodic techniques to systematically study the influence of system size on the convergence behavior of spin state energetics and Mössbauer parameters. The electronic structures resulting from periodic and molecular descriptions are compared in detail. Importantly, we find that several electronic structures are energetically accessible, some of which feature unpaired electrons in the carbon matrix. This degree of electronic complexity has not been established for FeNC catalysts so far. Furthermore, we demonstrate that iron high spin states are not energetically accessible in square-planar FeNC models, and show that axial distortions of the iron atom from the graphene plane or the binding of axial ligands are needed to explain the experimental observations.