Calculations of vertical excitation energies at the geometry configuration of the ON-state with the neutral chromophore (see Fig. ) at the XMCQDPT2 level result in the following values: 2.80 eV (442 nm, 2.1 oscillator strength (o.s.)) for the locally excited state transition S0 → S1, and 3.00 eV (413 nm, 0.6 o.s.) for the charge transfer transition S0 → CT. The computed transition S0 → S1 for the ON state conformation with the anionic chromophore (see Fig. ) corresponds to 2.46 eV (503 nm, 3.6 o.s.). These values for the S0 → S1 excitation are well consistent with the steady state absorption spectra for the ON-A protein form with the neutral Chro (411-413 nm) and the ON-B form with the anionic Chro (511 nm, higher intensity) . We conclude that the models for the ON-state is a good starting point for the consideration of the photo-induced hydration reaction.