The interplay between liquid–liquid and ferroelectric phase transitions in supercooled water

Significance Water has unique properties compared to other liquids. Understanding its behavior has significant implications in fields ranging from life science to meteorology. The hypothesis of a phase transition between two liquid states with different densities in the liquid metastable supercooled regime can explain some of water’s equilibrium anomalies. Analyzing molecular dynamics simulations, we show that dipolar degrees of freedom are involved in water’s liquid–liquid phase transition, providing a better overall understanding of its behavior. By developing a density functional theory in the mean field approximation holding for polar liquids, we further show that the liquid–liquid and the ferroelectric phase transitions are two sides of the same coin. This study significantly advances our understanding of water’s thermodynamic properties.