**4. Conclusions**

In conclusion, we use the anatase (211) surface as an idea model surface, containing one Ti4 and one Ti5 under-coordinated atoms in unit cell, to investigate their distinct properties. Our *ab initio* calculations show that the (211) surface is indeed a high reactivity surface with a high surface energy of 0.97 J/m2 . In addition, the four-coordinated Ti4 atoms with two unsaturated bonds have a stronger chemical reactivity in comparison to the Ti5 atoms with one unsaturated bond. Studies of water adsorption suggest two distinct states of adsorbed water on the (211) surface, one related to molecular water on Ti5 sites and the other to dissociated water on Ti4 sites. These results indicate that the Ti4 atoms will play a critical role in water decomposition. According to TiO2 structure, we propose a simple bond-charge counting model where each unsaturated Ti bond contributes 2/3 charge in average. As a necessary requirement of chemical reaction, the dissociation of water only occurs when Ti atoms provide more electrons to oxygen in water than H atom. Then, we reach to a conclusion that only Ti<sup>4</sup> atom or equivalent Ti4 can dissociate water. The controversy about whether Ti5 can dissociate water is resolved that Ti5 atom will eventually become Ti4 by breaking bond to neighboring O atom at surface with high surface energy while the dissociation will not happen for surface with low surface energy. Besides traditional DFT total energy calculation, this model is considered in a fundamental way. We can also declare that the bond charge offered by surface Ti atoms is the mechanism for water dissociation on TiO2 surface. Furthermore, the model is generic and applicable to both rutile and anatase surfaces including defects, e.g., step edges and vacancies.
