For model B, the ground state varies more in models of different size. While an intermediate spin state can always be found within a few kcal/mol of the ground state, the preferred electronic structure can also be described as an internal redox event, a ring-open-shell electronic structure, or a high spin configuration. The small energy differences between IS and HS state show that high spin states are energetically accessible in this type of model. We note that they may mix with intermediate spin states, as multireference character cannot be excluded. Overall, model B-2-MeHIm is more likely to explain the experimentally observed Fe(II) high spin signals, as was also concluded by Ni, Gallenkamp et al. With an axial Cl -ligand, the nominal ground state of the pyridinic models is more variable: with model sizes 1 and 2, an intermediate spin electronic structure is clearly favored, while at sizes 3 and 4 low spin becomes preferred. These low spin states are better characterized as pseudo-Fe(II) states resulting from an internal redox event. At model size 5, all three spin states are found within 1.5 kcal/mol of each other, high spin being the lowest. Nevertheless, the isomer shift predicted across all model sizes is ca. 0.4 mm/s and varies only within the uncertainty range. The quadrupole splitting for the low spin electronic structure is converged already at model size 3. The preferred high spin state at model size 5 has the same quadrupole splitting value as the intermediate spin states of model sizes 3, 4, 5 within the uncertainty range. Comparing the Mössbauer values predicted for both types of model with different axial ligands, it can be concluded that computational models can be used to distinguish different speciations of FeNC active sites. This mirrors previous findings in the literature, including our own. Next, we investigate the role of the 2-MeHIm and Cl -ligands on the spin state in our periodic model. For that 6×6 pyridinic and pyrrolic structures with the ligand bound to iron were fully relaxed using GGA and GGA+U. Binding an axial ligand results in lifting the iron atom from the carbon plane, though to different In the relaxed pyrrolic structure with 2-MeHIm, the iron atom is significantly elevated by 1.33 Å (0.80 Å) above the carbon plane, whereas in the relaxed pyridinic structure, it remains much closer at 0.34 Å (0.39 Å) using GGA+U (GGA). For the Cl ligand, the iron atom is displaced by 1.27 Å (0.86 Å) in the pyrrolic structure and 1.0 Å (0.33 Å) in the pyridinic structure using GGA+U (GGA). These different structures using different density functional theory approaches also lead to different predicted ground spin states. With the GGA approach, intermediate spin states are favoured. For the pyridinic structure with the 2-MeHIm ligand, M tot = 1.96 µ B is found, while the pyrrolic structure exhibits M tot of 2.24 µ B due to significant hybridisation with the nearest nitrogen atom in the ligand. Similarly for the Cl -ligand, a ground IS configuration with M tot of 3.0 µ B is found for the pyrrolic structure, while M tot of 1.05 µ B was calculated for the pyridinic structure. The GGA+U results indicate that all cases converge to a high spin state with M tot of about 4.0 µ B corre- sponding to 2S + 1 = 5 for the 2-MeHIm ligand. With the Cl - ligand, the ground state of the pyrrolic structure is found to have M tot = 5.00 µ B , while M tot = 3.12 µ B is obtained for the pyridinic structure.