  The local structure of liquid water plays a key role in determining the anomalous properties of water. We run all-atom simulations for three microscopic water models, and use multiple order parameters to analyse the local structure of water. We identify three types of local structures. In addition to the well known low-density-liquid and high-density-liquid structures, the newly identified third type possesses an ultra high density and overcoordinated H-bonds. The existence of this third type decreases the rate of transition from the high-density-structure to low-density-structure and increases the rate of the reverse one, leading to the enhancement of the high-density-structure stability. 
Ultra-high-density local structure of liquid water