To gain insight into this difference in the orbital occupations, we analyzed the non-spin-polarized GGA results, excluding the effects of magnetism. Figure dicates that potential interactions between neighboring iron atoms are minimized. The d xy orbital is strongly destabilized with a huge gap of about 3eV due to the pdσ -antibonding interaction between Fe d xy and N p x/y orbitals, and the d x 2 -y 2 orbital is always the most stabilised. Considering the rotation of the axis system with respect to the molecular models, this is in full agreement with the results presented in the previous section with the molecular model. For the pyridinic case, the energetic sequence, and hence the orbital occupation pattern, of the d yz , d z 2 and d xz orbitals is similar to the molecular models. In the pyrrolic model, a different energetic sequence is found in the periodic model compared to the molecular model, specifically the d z 2 orbital being more stabilized. This leads to an IS1 ground state for the periodic model compared with an IS3 ground state in the molecular model. Since the iron and nitrogen distances differ by only 0.017 Å between the periodic models with pyridinic and pyrrolic nitrogen donors, we trace this difference to the enforced planarity of the periodic models. More specifically, nuances in the ligand field splitting of the iron 3d orbitals likely stem from the surrounding carbon environment given the high structural similarity of the immediate FeN 4 units. Therefore, the two distinct nitrogen environments appear to generate different ligand fields, leading to different orbital occupations of the iron 3d orbitals with an overall 2S + 1 = 3 intermediate spin configuration, see Figure .