Extensive quantum chemical DFT calculations were performed on the high-resolution (1.9 angstrom) crystal structure of photosystem H in order to determine the protonation pattern and the oxidation states of the oxygen-evolving Mn cluster. First, our data suggest that the experimental structure is not in the S-1-state. Second, a rather complete set of possible protonation patterns is studied, resulting in very few alternative protonation patterns whose relevance is discussed. Finally, we show that the experimental structure is a mixture of states containing highly reduced forms, with the largest contribution (almost 60%) from the S-3-state, Mn(II,II,III,III).(1)