Comparing results for the inner pair in vacuum and in the protein environment as seen in Figs. ) and d), a redistribution of the spin density from P A to P B in the latter case can be observed. The resulting spin-density ratio in the binding pocket is equal to 24%/76% in favor of P B . This strong localization at the P B co-factor is in good agreement with experimental EPR and ENDOR measurements , where 75% to 100% of spin density was found at the P B co-factor. The calculated ratio in the binding pocket also agrees very well (deviations within 1%) with previous theoretical computations reported in Ref. and supports the assumption that asymmetry in the inner pair of PSI is induced by the surrounding protein environment. As can be seen from Figs. ). This observation cannot be validated by comparison with experimental results as, to the best of our knowledge, spin population data is currently unavailable in literature for the BRC. However, the surrounding protein environment does influence the resulting spin-density ratio. Thus, the ratio changes from 68%/32% for the dimer in vacuum to 93%/7% for the SP in the protein binding pocket, showing much stronger localization at the D A co-factor in the latter case. This result is rather surprising taking into account that ENDOR and TRIPLE experiments as well as previous KS-DFT calculations predict 66% and 60% of spin density to be localized at D A , respectively. However, it agrees with photo-CIDNP experiments reporting a strong localization of the spin density in the SP of Rhodobacter sphaeroides . A definite conclusion on the correct spin-density localization in the SP of the BRC cannot be drawn on the basis of the presented results and would require much larger molecular In this work, we presented an application of the recently developed FDE-diab approach to RC models from PSII, PSI, and purple This metodology allowed us to avoid the consequences of the DFT overdelocalization error in the intermolecular regime and to reliably calculate spin distributions and electronic couplings for a number of RC models.