Very recently Umena et al. have determined the X-ray diffraction (XRD) structure of the CaMn4O5 cluster (1) in the oxygen-evolving complex (OEC) of photosystem II (PSII) refined to 1.9 angstrom resolution. UB3LYP calculations of 1 using this XRD structure were performed to elucidate possible mechanisms for the oxygen-oxygen (O-O) bond formation in oxygen evolution reaction of PSII. The solutions obtained were used for natural orbital (NO) analysis to obtain LUMOs of labile chemical bonds, which clearly indicated the possibilities of nucleophilic attack of hydroxide (or water molecule) to the electrophilic O(56) and/ or O(57) sites of 1. (C) 2011 Elsevier B. V. All rights reserved.