The oxygen-evolving complex (OEC) of photosystem II is the only natural system that can form O2 from water and sunlight and it consists of a Mn4Ca cluster. In a series of publications, Siegbahn has developed a model of the OEC with the quantum mechanical (QM) cluster approach that is compatible with available crystal structures, able to form O2 with a reasonable energetic barrier, and has a significantly lower energy than alternative models. In this investigation, we present a method to restrain a QM geometry optimization toward experimental polarized extended X-ray absorption fine structure (EXAFS) data. With this method, we show that the cluster model is compatible with the EXAFS data and we obtain a refined cluster model that is an optimum compromise between QM and polarized EXAFS data. © 2012 Wiley Periodicals, Inc.