Fig. S1.

The whole QM/MM region used for calculation. The MM and QM regions are expressed in wire and ball/stick styles, respectively. Atoms of amino acid residues, water molecules, the Mn₄CaO₅ cluster, and Cl⁻ ions within 20 Å from the Mn₄CaO₅ were taken from the XFEL X-ray structure of PSII (Protein Data Bank ID code 4UB6) (5), and hydrogen atoms were generated and optimized using AMBER.