Lacombat et al. speculate that X might be an adduct of the anionic Chro with the water molecule. The results of our simulations suggest that X can be described as an adduct of the water molecule with the cationic Chro and anionic Tyr203 complex. According to the present QM/MM calculations, the energy of the X intermediate is 27 kcal/mol above the level of the initial ON-state structure. We find that the barrier separating the X-and OFF-structures is low, not exceeding 1 kcal/mol. The level of the OFF-state structure is 6 kcal/mol above the ON-state level, consistently with the observed process of the thermal recovery of the fluorescent state in Dreiklang as well as with the results of previous simulations . The computed excitation energy at the X point 2.79 eV (444 nm) agrees well with the observed transient absorption band maximum at 450 nm reported in the femtosecond spectroscopy study . Table . Comparison of the computed S0 → S1 excitation bands and experimental absorption bands. Structure ON-A refers to the protein in the ON-state with the neutral Chro (Fig. ); structure ON-B refers to the ON-state with the anionic Chro (Fig. ). summarizes the results for the absorption band maxima at the stationary points on the ground state PES obtained in computations using the XMCQDPT2 method. Comparison with the experimental data provides a strong support to simulations. We conclude that the present simulations are consistent with the structural and spectral experimental observations, thus validating the suggested mechanism of the light-induced reaction in Dreiklang.