Based on the structure of 1.9 angstrom resolved OEC, which was published as PDBid = 3ARC, geometries and electronic structures of the CaMn4O5 core and surrounding water molecules at the S-0 state are examined by the hybrid B3LYP and broken-symmetry methods. Two doublet states 1 and 2 with energy difference of 15.2 kcal/mol were obtained. 1 is lower than 2. The oxidation states of four manganese atoms of 1 are Mn-4[III, III, IV, III], while those of 2 are Mn-4[III, IV, IV, II]. It is concluded that the oxidation states of Mn-4(II, III, IV, IV) is ruled out at the S-0 state. (C) 2012 Elsevier B.V. All rights reserved.