Minimization for the SA2-CASSCF second root initiated from the Frank-Condon point arrived at the structure shown in Fig. . In this computational scheme, the CT minimum lies about 1 eV lower than the S0 → CT excitation level. In comparison with the structure at the S0 minimum (Fig. ), the reactant water molecule Wat stays notably closer to the tyrosine species and the imidazolinone ring (I-ring) of the chromophore. Subsequent evolution of the CT excited state system is associated with the proton transfer along the chain Tyr-Wat-Chro (see Fig. ). Estimates of the potential barrier from the located CT minimum give the value not exceeding 2 kcal/mol.