To further support the relative sign and magnitude of the coupling constants obtained, we have performed DFT calculations (see the SI for the computational details) on a model complex, 1A, derived from complex 1 (Figure ). These suggest that the eight independent exchange interactions are in the range |J| = 4.6-16.4 cm -1 (Table ), in good agreement with the experimental simulations. All are antiferromagnetic in nature, with the exception of the Fe4-(µ4-O 2-)3-Fe3 interaction which affords J = +4.6 cm -1 on account of the large Fe-O bond lengths and small Fe-O-Fe bond angles present which lead to orbital orthogonality. Overlap integral calculations using metal-based singly occupied molecular orbitals (SOMOs) reveal that the strongest antiferromagnetic interactions occur where there are a higher number of strong or moderate overlap integrals, and vice versa (Fig. ). For the Fe4-(µ4-O 2-)3-Fe3 interaction (J1 in Table -4) there is only one strong interaction (dz 2 ||dxz) with the remaining 24 interactions being weak. The overall result is a weak/moderate ferromagnetic interaction. Spin density analysis suggests that strong spin delocalisation is present in 1 with Fe III spin densities ranging between 4.007-4.151 (Fig. ).