Based on the structure at 1.9 angstrom resolved OEC, which was published as PDBid=3ARC this year, the electronic structures of the CaMn4O5 cluster at S-0, S-1, and S-2 states are examined under the geometry optimization of the mixed-valence CaMn4O5 cluster and water molecules. It is shown that optimizing geometry purifies the spin structures of four manganese atoms, easily determining their oxidation states. It is also shown that one of two water molecules is a substrate for the oxidation to yield the oxygen molecule.