To complete the reaction, we return to the full-protein model and consider the structures X (the reaction intermediate) and OFF (the reaction product) at the ground state PES. To this goal, the coordinates obtained at the SA2-CASSCF level (see panels d, e in Fig. ) were incorporated into the protein model and re-optimized at the QM/MM level. Figure illustrates the structure in the OFF-state. The panel (a) shows the active site; the modified chromophore contains the corrupted I-ring with the covalently bound hydroxyl group; the latter is hydrogen-bound to the Glu222 side chain and the main chain fragment of Leu68. Panel (b) shows superposition of fragments of the active site in the QM/MM optimized structure (colored balls and sticks) and in the crystal structure PDB ID 3ST3 (yellow stick). It is important to note that we did not use coordinates of the experimental structure PDB ID 3ST3 as input data for simulations. Instead, we arrived to this construct in the course of simulated chemical transformations initiated from the ONstate structure. An agreement between simulation results and experimental data for the OFF-state, which is of similar quality as for the starting ON-state, provides the support to our simulations.