The computed with the XMCQDPT2 method excitation energy at the located point (3.57 eV, 347 nm) also well agrees with the experimental absorption band maximum (3.65 eV, 340 nm) . The main part in Fig. shows the computed energy diagram for the ON → OFF reaction with the energy values at the ground state structures ON, X and OFF computed at the QM/MM level. The most important fragment of structure X optimized in the full-protein model is shown in the inset in Fig. . The reactant water molecule Wat stays close to the target I-ring ready for the final elementary step, i.e. an attack of the hydroxyl ion from Wat on the C65 atom in the I-ring coupled with the proton transfer to Tyr203. Although we keep the same notation X for the ground state reaction intermediate as suggested in Ref. , its nature differs from the initial proposal.