Understanding the WOC's mechanism of low energy oxidation of water to molecular oxygen, has been the subject of a wide number of spectroscopic, crystallographic, and computational studies. The penultimate S3 state is a major current focus of contemporary research as it is the last semi-stable intermediate before oxygen evolution and has been associated with the oxygen-oxygen bond formation step. The characterisation of this state was greatly enhanced by the recent report of an atomic-level Xray crystal structure of the WOC using X-ray free electron lasers (XFEL), after two light flashes (2F) and poised predominantly in the S3 state. The core structure obtained from these studies and the main atom numbering is shown in Figure . High frequency continuous wave Electron Paramagnetic Resonance (EPR) and ELDOR-detected NMR (EDNMR) spectroscopy studies have shown that the S3 state gives rise to an S = 3 EPR spectrum for the cyanobacterum Thermosynechococcus elongates. The analysis indicated that the four Mn ions are present in the IV oxidation state and octahedrally coordinated. This was supported by broken symmetry density functional theory (BS-DFT) and Heisenberg-Dirac-van Vleck (HDvV) spin ladder calculations. XFEL crystallographic studies have shown that an O6 atom is detected in the 2F structure in very close proximity to O5. The presence of an oxo-oxyl bond, [O2] 3-or an equilibrium of these forms has been proposed to describe this. Recently, a five-coordinate precursor S3 state was suggested to explain broadened EPR signals in modified forms of the 2F state relative to the native form. This five-coordinate intermediate, remaining an S = 3 system, was proposed to feature a closed cubane (hexacoordinated Mn1 and pentacoordinated Mn4) rather than an open cubane form (both Mn1 and Mn4 hexacoordinated) reported previously for the native system, supporting a pivot or carousel mechanism of water oxidation where Mn4 is the binding site for a new water or hydroxide in the S2 to S3 transition. Similarly broadened EPR signals have been reported by Marchiori et al. on glycerol treated samples, attributing these, by contrast, to an open cubane form. More recently it was proposed, based on BS-DFT calculations of hyperfine coupling constants (hfcs) and zero-field splitting (D) values, that an open cubane architecture was preferred for both native and alcohol-modified forms. Assignment of spectral bands from EDNMR data to individual hfcs relies on accurate spin Hamiltonian simulations of both the relative intensity and position of spectral peaks in experimental EDNMR spectra. At high frequency, EDNMR has become the technique of choice for detecting nuclear transitions in the WOC. This is achieved by employing a long, low powered high-turning angle (HTA) microwave pulse to drive polarisation transfer between formally forbidden electron transitions (ΔmS = ±1; ΔmI = ±1), which is subsequently detected as a change in the echo intensity of a simple Hahn Echo or Free Induction Decay (FID) detection sequence. EDNMR is advantageous over analogous hyperfine techniques, namely Electron Nuclear DOuble Resonance (ENDOR) spectroscopy, as neither radiofrequency (RF) amplifier bandwidth limitations nor microwave pulse selectivity pose a problem in data collection or analysis, as is the case in Davies ENDOR. Moreover, EDNMR is robust against fast longitudinal relaxation (T1). Previously, EDNMR spectral simulations have been performed by approximating the experimental EDNMR spectrum as an ENDOR spectrum using the salt routine in EasySpin. In this report by contrast, we take advantage of the recently reported routine horseradish, also implemented in EasySpin and described in detail elsewhere, but in summary, the horseradish simulation considers all allowed and forbidden transitions for a given spin system and calculates the detection probability and inversion efficiency of each centre by identifying connected pump transitions, i.e. those related to the frequency of the HTA pulse, giving a more accurate assignment of the spectrally observed EDNMR bands. Importantly, the central blind spot of the EDNMR experiment, where the HTA pulse saturates the detected EPR transition, centred around υHTA -υObs = 0 MHz, is not modelled explicitly in these calculations.