Due to the low barrier between Fe 2+ and Fe 3+ and nearly degenerate low-energy electronic states they catalyse important redox reactions and can serve as charge or molecular carriers. The relative stability of the low-energy spin states depends on ligand field and many-body correlation effects experienced by the metal center, that, in turn, depend on chemical functionalization and geometry of the conjugated macrocycle. For this reason, a reliable theoretical prediction of the energetically low-lying spin states is challenging and necessary to facilitate the understanding of nature's efficient enzymatic reactions.