show very similar results compared to their truncated counterparts. It is interesting to note that deprotonated models comprising the P A co-factor show about three times larger values of both electronic couplings |V | and energy differences |H 11 -H 22 | than those with P B . This, however, does not influence the resulting spin-density distributions. Another aspect that should be noted is the magnitude of the energy gap |H 11 -H 22 |, which is much larger than the gaps calculated for the deprotonated hinge models of PSII. In the latter case, the energy levels H 11 and H 22 were nearly degenerate and, therefore, changed their ordering with modifications in the mutual orientation of fragments (see Sec. 4.1). This does not occur in deprotonated models of PSI, where the initial ordering of the electronic ground and first excited state remains unchanged. The interaction with the surrounding protein slightly changes the spin-density distribution resulting in the ratio of 94%/6% (see Tab. S11 in the SI). Therefore, the obtained results for both protonated and deprotonated complexes in PSI show only a minor degree of the spin-density delocalization between Chl and His molecules. This agrees well with experimental photo-CIDNP NMR studies , where all spectroscopic signals were attributed to either a single Chl a molecule or the [P A • • • P B ] dimer . Unfortunately, this small degree of delocalization in the calculated complexes prevents us from drawing an unambiguous conclusion about the protonation state of the axial His molecules in PSI.