Our starting point on the pathway to O5-O6 bond formation is an O5 oxo O6 hydroxo form of the WOC complex, Figure , formed after initial formation of the S 3 state. This corresponds to the S=3 form detected by EPR with four Mn (IV) ions. O6 corresponds to the new oxygen atom detected by XFEL after the second flash. For this oxo-hydroxo model seven broken symmetry, Ms, states are possible at the optimised geometry. We have shown that two of these, both M S =3, [Mn4(↓↓↓)Mn3(↑↑↑)Mn2(↑↑↑)Mn1(↑↑↑)] and [Mn4(↑↑↑)Mn3(↓↓↓)Mn2(↑↑↑)Mn1(↑↑↑)] are the lowest in energy and govern the spin density of the complex resulting in a spin distribution of close to 0.5, 0.5, 0,0 and 0.0 for Mn1-Mn4. This explains the set of two large (Mn1 and Mn2) and two small (Mn3 and Mn4) magnitude Mn hfcs observed using EDNMR spectroscopy. For the oxo-hydroxo form no changes are found in the IBOs in the region of 2.5 -2.0 Å and as mentioned above this model becomes unstable at bond distances less than 2.0 Å. By contrast significant changes are observed for both oxo-oxo forms. The IBOs which undergo significant changes are located by monitoring the root-mean-square deviation of every IBO from the initial partial charge distribution along the PES. Figures and identifies four main IBOs participating in bond making and breaking during the reaction. These are, the α and β spin orbitals of the Mn 4 O5 σ-bond, the β spin orbital of the Mn 1 O6 σ-bond and the α spin orbital of one of the π-bonding lone pair orbitals on O6. As the O5-O6 bond distance is decreased from the non-bonded oxo-oxo form, the α electron density of the Mn 4 O5 sigma bond evolves into a dz 2 orbital on Mn 4 , Figures and (pink), at an O5-O6 bond distance of around 2.2 Å for Ms=6 and 2.0 Å for Ms=3. Concurrently with this electron density rearrangement, the α density of the π-lone pair on O6, evolves to a σ bond between the O6 and O5 oxygens, Figures and (blue). A Mayer bond order analysis, Figure , also illustrates such a change with a decrease in the Mn 4 -O5 bond order from near 1.0 to near 0.5 and an increase in the O5-O6 bond order from 0.0 to near 0.4. In a similar fashion, Mulliken spin population analysis, Figure shows a change in spin population of Mn 4 from near 3.0 to 4.0 signalling a reduction from Mn 4 (IV) to Mn 4 (III). For the Ms=3 state, further progression along the PES shows the β-electron of the Mn 4 -O5 σ-bond evolves to become the β component of the O5-O6 σ-bond, Figure (yellow), and the Mn 1 -O6 σ-bond β electron density evolves into a dz 2 orbital on Mn 1 , Figure (green). For Ms=6, the β-electron of the Mn 4 -O5 sigma bond again evolves to become the β component of the O5-O6 sigma bond, Figure (yellow), while in this case a Mn 1 -O6 π-bond β electron density evolves into a dπ orbital on Mn 1 , Figure (green). Mulliken spin populations, Figure , correspondingly show an increase in spin population from 3 to 4 for Mn1 illustrating reduction of Mn1 to high spin Mn(III) for Ms=3 whereas for Ms=6 the electron transfer of a β electron to Mn1 results in occupation of a dπ orbital, Figure (green), resulting in a spin population of 2 and corresponding to a low spin form of Mn(III). The Mayer bond order values for both Ms states of Figure show an increase in the O5-O6 bond order to near 1.0 as the peroxo is formed.