Ultra-high-density local structure in liquid water*

We employ multiple order parameters to analyze the local structure of liquid water obtained from all-atom simulations, and accordingly identify three types of molecules in water. In addition to the well-known low-density-liquid and high-density-liquid molecules, the newly identified third type possesses an ultra-high density and over-coordinated H-bonds. The existence of this third type decreases the probability of transition of high-density-liquid molecules to low-density-liquid molecules and increases the probability of the reverse one.