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The step where two oxygen atoms join to make $\\mathrm{O_2}$ has not been directly seen.","posed_since":"","precise":"For the $\\mathrm{Mn_4CaO_5}$ oxygen-evolving complex of photosystem II, identify the O-O bond-forming step in the S3 to S4 to S0 transition: which oxygen atoms couple (O5 with the inserted O6, or a water-derived ligand), the mechanism (oxyl-oxo radical coupling or nucleophilic attack), the Mn oxidation and spin states at the transition state, and its barrier consistent with the measured millisecond kinetics. An answer is a mechanism supported by time-resolved structural and spectroscopic data and by converged electronic-structure calculations.","problem_ref":"5dfd896fe59749c2c4830ce233fb2408fd8089f6d1b2f62bf9634dec153b913b","references":"","settled_by":"Time-resolved crystallography and X-ray emission spectroscopy capturing the S4 intermediate, matched by multireference calculations of the bond-forming step.","status_note":"In 2023 time-resolved crystallography (Bhowmick et al., Nature, https://doi.org/10.1038/s41586-023-06038-z) resolved intermediate structures in the $S3$ to $S0$ transition; the identity of the bond-forming step is still debated (not rechecked for 2025-2026).","title":"How the O-O bond forms in photosystem 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