, where A is P A , P D1 or D A , while B is P B , P D2 or D B . The case of truncated radical cation models in the electronic ground state is summarized in Fig. , while the results for their non-truncated counterparts are given in Fig. . As can be seen from Fig. (top), inner pairs of PSI, PSII, and BRC show very different spindensity distributions. At ∆R ⊥ = 0 Å, the ratios are equal to 96%/4%, 80%/20%, and 72%/28% for the inner pairs in PSI, PSII, and the BRC, respectively. Hence, FDE-diab predicts most of the spin-density to be localized at the P A , P D1 , and D A co-factors of the corresponding pairs. The results obtained for PSII and the BRC are in a very good agreement with experimental measurements , where 82% and 66% of the spin-density were found to be localized at P D1 and D A , respectively. In case of PSI, however, most of the spin density is expected to be localized at P B , while the calculated ratio shows the opposite result. This can probably be explained by the inability of FDE-diab to reproduce the correct order of nearly degenerate electronic states in PSI, which are separated by only 0.14 eV. The spin-density ratio in the first excited state of the PSI inner pair is equal to 4%/96% (see Fig. ) and, thus, shows the anticipated strong localization at P B . Another reason for the exchanged order of electronic states could be the complete neglect of co-factor-protein interactions in the presented FDE-diab calculations. The results obtained for non-truncated models are generally very similar (see Fig. ) to those