The quantum-chemical assessment of such models is an extremely challenging task as it usually requires considering the nearby protein environment. Hence, the choice of a quantum chemical method is practically limited to the standard Kohn-Sham Density Functional Theory (KS-DFT) or its variations such as, for example, Frozen-Density Embedding (FDE) . However, KS-DFT is known to suffer from the self-interaction error (SIE) , which leads to overdelocalized spin densities . Although reliable spin distributions can often be obtained for single molecules (for example, see Ref. ), molecular systems containing non-covalently bonded fragments, similar to those in RCs, could be especially problematic and lead to inconclusive results . A methodology allowing to effectively avoid this limitation was reported in our recent work and called FDE-diab . It was tested on model molecular clusters featuring different degrees of spin-density delocalizations and compared to accurate ab initio computations . More recently, it was also applied to inner pair co-factors in RCs of PSI, PSII, and purple bacteria, and verified against previously available experimental measurements . In both cases, a good agreement with theoretical and experimental results was found proving that the suggested approach is robust and effective. It should also be noted that this approach allows to consider much larger molecular systems then accessible with KS-DFT. This makes it a very attractive tool for investigations of photosynthetic pigment models in RCs.