Shifts of the special pair redox potential of photosynthetic reaction centers of Rhodobacter sphaeroides for five different mutants are considered. The shifts are calculated by the method of free energy perturbation which is based on dynamics simulation data. The influence of the orientation and the torsion potential of the acetyl group on the shift,of the redox potential is investigated and compared with recent results obtained by solving Poisson’s equation. Int the oxidized state of the special pair the acetyl group at DL reorients in the initial part of the dynamics simulation. As a consequence, the hydrogen bond with H(L168) breaks and the oxygen atom binds to the Mg2+ ion of DM. For I:he mutant EY(M197) there is evidence that the acetyl oxygen atom of D-M binds to the Mg2+ ion of DL instead of forming a hydrogen bond with Y(M197).