The oxidation of water to oxygen,2H2O → O2+4H++4e-,is a key step for capturing solar energy in nature.Developing a molecular catalyst that can split water into oxygen and hydrogen is one of the main bottlenecks inhibiting the development of an effective and robust artificial photosynthetic system.Although a large body of molecular transition-metal catalysts and active metal oxide materials have been developed for water oxidation,substantial challenges remain for the ultimate goal of an efficient,inexpensive and robust electro/photocatalyst.The main challenge for catalyst development is that the catalytic process involves accumulative proton coupled electron transfer,multiple bond arrangements and finally the formation of O-O bond.Many molecular transition-metal complexes have been developed as water oxidation catalysts(WOCs),but clearly most current approaches to the design of WOCs aren't works comparable with Mn4Ca cluster in natural system.Generally,a high energetic intermediate,Mn+=O,was essential to O-O bond formation.On the other hand,the formation of high energetic intermediates needs to be avoided as they can reduce the longevity of the WOC and make the operation potential far way from the thermodynamic potential of water oxidation.In PSⅡ,Mn4Ca cluster distributes charge over multiple metallic centers to avoid charge accumulation on single site and formation of Mn(Ⅴ)that was proposed as key intermediates for artificial molecular Mn based WOCs.