Inherent structures of water using fully ab initio simulations

Using fully ab initio molecular dynamics simulations based on the SCAN functional, we study the inherent structure of water and its temperature dependence. Our results show three types of translational ordering of the second oxygen coordination shell. With this as a criterion, the local structures in water are classified into three types, here named structures I, II, and III. In structure I, the second shell loses its translational ordering, while in structures II and III it presents a translational ordering similar to that in ice II (or ice V) and ice III, respectively. However, the tetrahedral orientational ordering distribution and bond-angle distribution in structures II and III are different from those in ice II (or ice V) and ice III. This indicates that the local atomic structures of liquid water and crystalline ice are not identical, although they have similar translational ordering. The temperature dependence of the inherent structure suggests that the density maximum of water originates from the competition not only between structures I and III, but also between structures II and III. These results provide fully ab initio evidence for the mixture model of water.