The oxygen-evolving complex of photosystem II catalyzes one of the most challenging reactions in biology. Information on how the electronic structure of the tetramanganese cluster of the OEC evolves and transforms during the catalytic cycle is essential for understanding the water oxidation mechanism on a microscopic level. Here, I will discuss how quantum chemical techniques connect geometric structure with electronic structure and spectroscopy, evaluate structural models proposed by crystallography, and elaborate on possible mechanistic scenarios for further experimental investigations. Analysis of the localization of electrons on individual Mn ions and consideration of the total spin states of the exchange-coupled cluster are essential for these tasks. Our results expose limitations of current X-ray free electron laser crystallographic models of the oxygen-evolving complex in photo-advanced intermediate states,1 while quantum chemistry in combination with X-ray absorption spectroscopy highlight specific formulations of the manganese cluster with direct implications for our understanding of catalytic progression.2 Explicit consideration of spin coupled states in connection to electron paramagnetic resonance observations, as well as utilization of state-of-the-art correlated wavefunction approaches, shed light into the finer details of metallocofactor plasticity, on the spin dynamics that control S-state advancement, and on the sequence of events that set up the cluster for O-O bond formation.3-5