FDE-diab calculations of isolated inner pair models from PSI predicted about 96% of spin density being localized at P A . Although the exact spin-density ratio for the inner pair of PSI is unknown, various experimental measurements (for example, see Refs. ) predict from 75% to 100% of the spin density to be localized at the P B cofactor. Therefore, the FDE-diab calculations for isolated inner pair models contradict experimental results. This, however, is caused by a complete neglect of the co-factorprotein interactions. Inclusion of the protein environment into FDE-diab calculations changed the spin-density distribution considerably and resulted in the ratio of 24%/76% in favor of P B , which is in good agreement with available experimental results. We also managed to reproduce most of experimentally measured P B spin populations using the large protein binding pocket model. However, it turned out to be difficult to reliably calculate some small atomic spin contributions.