Large-scale QM/MM calculations including hydrogen-bonding networks in the oxygen evolving complex (OEC) of photosystem II (PSII) were performed to elucidate the geometric structures of the CaMn4O5 cluster in the key catalytic states (S-i (i = 0-3)). The optimized Mn-Mn, Ca-Mn and Mn-O distances by the large-scale QM/MM starting from the high-resolution XRD structure were consistent with those of the EXAFS experiments in the dark stable S-1 state by the Berkeley and Berlin groups. The optimized geometrical parameters for other S-i (i = 0, 2, 3) states were also consistent with those of EXAFS, indicating the importance of the large-scale QM/MM calculations for the PSII-OEC. (C) 2016 Elsevier B.V. All rights reserved.