To complement the analysis above based on molecular models, periodic models were constructed in three sizes (n × n with n = 4, 6, and 8), and then fully relaxed. We note that the periodic models have an overall neutral charge. As was shown with the molecular models, the relevant IS configurations of A and A 2+ behave very similarly, especially those that are spectroscopically discernible. When using periodic boundary conditions, one needs to examine the size of the unit cell to minimize the interaction between two adjacent iron atoms. In the 4×4 model, the distance between two iron atoms amounts to 9.73 Å (10.11 Å) in the pyridinic (pyrrolic) structure. Inspection of the spin density near the unit cell boundary suggests that this distance is too short to avoid iron-iron interactions, especially in the pyrrolic case, see Fig. and). In the 6×6 model where the neighboring iron distance increases to 14.70Å (14.96 Å), the spin density around the iron atom no longer extends to the unit cell boundary. As shown in Fig. , enlarging the unit cell significantly reduces the bandwidth of both d yz and d xz orbitals, as discussed further below. This trend continues for the 8×8 model, so that a unit cell of at least size 6×6 can eliminate potential iron-iron interactions in periodic models.