Broken symmetry density functional theory (BS-DFT) calculations combined with ELDOR-detected Nuclear Magnetic Resonance (EDNMR) spectral simulations have been used to assign the 55 Mn hfcs for modified spectral forms of the water oxidising complex (WOC) in the penultimate S3 state of the water oxidation cycle. The analysis indicates that an open cubane form of an all-octahedral Mn ion, oxo (O5)