We distinguish three lowest singlet state energy levels at the structure of the ON-state with the neutral Chro (Fig. ) denoted S0, S1 (local excited state), CT (charge-transfer state). As shown in Ref. the levels of the S1 and CT states at this geometry are almost isoenergetic, and their precise location depends on the applied theoretical level. Within the CASSCF framework, the first two roots refer to the S0 and CT states, whereas the S1 state appears as the third root. Therefore, the use of a practical state-average approach SA2-CASSCF allows us to explore the relief of the potential energy surface in the CT state, which finally leads to the hydration reaction pathway as illustrated in Fig. . In the panels in Fig. , we show only four molecular groups, Chro, Wat, Tyr203 and Glu222. For better visibility, the shuttling protons, which initially reside on Tyr203 and Wat, are distinguished by the cyan and pink colors.