Suga et al first reported an O5-O6 bond distance of 1.5 Å indicating peroxo formation. Later studies proposed an additional oxygen O x similar to O6 of the Suga et al structure but with an extended O5-O6/O x bond length of 2.1 Å. More recently Suga et al propose a best fit O5-O6 bond length of 1.9 Å. All structures of S 3 so far appear to rule out an oxohydroxo non-bonded form which requires an O5-O6 bond separation of at least 2.5 Å. The interpretation of the S=3 signal EPR from the 2-flash state based on BS-DFT analysis of calculated hfcs is highly indicative of an oxo-hydroxo form for the S 3 state. An oxohydroxo model is not however compatible with the structure obtained by XFEL. As described above the [O5O6] 3-model does agree with the XFEL structures. This corresponds to a broken symmetry Ms=6 spin state. This is not a true spin state, S. The true spin state energies can be obtained by diagonalization of the Heisenberg Dirac van Vleck Hamiltonian using J values obtained by analysing all possible BS states. Table shows the calculated J values and energies of the ground spin states using this procedure. From this an S=6 spin state is calculated to be the ground state spin. This therefore cannot be attributed to the species observed by EPR/EDNMR which has an S=3 ground state spin. The PES shows that the two species are related by the protonation state of O6. Intriguingly an S=6 species was proposed to be formed in the 2-flash S 3 state of spinach samples and was proposed to be the major component (80%) of native samples. The S=6 form was attributed to a so-called closed cubane form of the WOC cluster with a pentacoordinated Mn 4 (IV) ion formed before the second substrate binds. So far no structural experimental support for such a closed cubane structure of the WOC has been obtained for any S-state. It is therefore more likely (see below) that this species corresponds to the [O5O6] 3-form alluded to in this manuscript, also with S=6. Experimentally, no S=6 species has so far been reported in cyanobacteria samples where high resolution high field W-band EPR spectra obtained are attributed to an S=3 form Simulations of the W-band EPR spectra for the S=3 form are shown in Figure . Also shown are simulations for an S=6 form using the zero field splitting parameters reported for the spinach samples. From the simulations it is clear that the spectral intensity of the S= 6