The MD study of water short-range order during liquid-liquid transition: Toward the second critical point

Full-scale thermodynamic experimental investigations of the liquid-liquid transition and different water phases remain challenging due to spontaneous crystallization and the complexity of studies on the nanosecond time scale. Moreover, the specific position of the second critical point in water is still unclear. This study used molecular dynamics to reproduce the structural properties of both low- and high-density waters and to characterize changes in the order parameters, average number of hydrogen bonds, and ordering of neighboring molecules during liquid-liquid transitions and in regimes of supercooled waters. A rapid change in calculated parameters under increased pressure could be attributed to a first-order phase transition. In addition, we provided evidence that low-density water becomes dissolved in high-density water inclusions as the temperature or pressure increases. The presumed location of the second critical point has been identified as TC ≈ 220 K, PC ≈ 1.5 kbar, and ρC ≈ 1.02 g/cm3.