Tabs. S3 to S4 in the SI) with a subsequent redistribution of the calculated spin density between the two molecules. The ground state spin-density ratio of 86%/14% obtained for the non-truncated [P D1 • • • His] • complex is in excellent agreement with the ENDOR measurements from Ref. , which predict 82% of the spin to be localized at P D1 . However, based on these findings we cannot discard the possibility that the spin density might be delocalized between all three fragments: His, P D1 , and P D2 . Our recent FDE-diab calculations carried out for the inner pair molecules in Ref. showed a considerable electronic coupling between P D1 and P D2 with the resulting spin-density ratio of about 80%/20% for these molecules. It should be noted, however, that in FDE-diab calculations reported in Ref. , MOs of His were not used for constructions of quasi-diabatic states, thus (by construction) preventing the spin density from delocalizing over the His residue.