Catalytic conversion of solar energy into chemical energy has been frequently investigated to develop dean energy sources in the last few decades. Metal oxide complexes show high potential for the catalytic conversion process, but the biochemical process in green plants has better efficiency than artificial photocatalysts consisting of metal oxides. In this work, the water splitting process is theoretically investigated using two synthetic model complexes whose structures are similar to the manganese-based oxygen evolving complex in photosystem II. Model A consists of four Mn atoms, and model B consists of three Mn atoms and a Ca atom in the core. Model A shows a better ability for water splitting than model B when comparing the highest reaction energy. The highest reaction energies are 2.56 and 2.99 eV for models A and B, respectively. In model B, the first oxidation in the water splitting process is exothermic, which is different from model A. In both models, the molecular oxygen generation step is endothermic by about 1.0-2.5 eV.