Direct observation of liquid–liquid phase coexistence in deeply supercooled water using an accurate polarizable multipole model

Liquid water can be supercooled up to about 50 K below the melting point before undergoing homogeneous ice nucleation. Based on experimental thermodynamic observations and computer simulations, it was hypothesized that below this temperature and at pressures of several kbar, water undergoes a liquid-liquid phase transition (LLPT) and the transition line ends at a second critical point. However, challenges in experiments and simulations at such deep cooling leave doubts about the nature of the LLPT and the existence of the critical point. Here, we use molecular dynamics simulations with a highly accurate and computationally efficient polarizable water model to establish the character of the LLPT and identify the location of the second critical point. Our microsecond-long simulations provide direct evidence of a well-defined moving interface between low-density and high-density water at conditions near the phase boundary. This provides decisive proof of a first-order transition between two liquid phases with distinct free energy basins separated by a barrier, taking a major step toward resolving this long-standing debate. These results offer new perspectives on supercooled water under pressure simulated with an accurate and realistic model suitable for studies of water in confined geological and biological environments.