Unveiling enigmatic phase transitions of water in the supercooled region and no man’s land

In the perplexing domain of supercooled water, scientists have dedicated decades to unraveling its mysteries. Using advanced sampling methods within the TIP4P/2005 water model, this study pinpoints the second critical point at temperature Tc= 238±2 K and pressure Pc= 288±30 bar, signifying an abrupt first-order to a gradual continuous phase transition. It also reveals a transition temperature Tt=172±1 K at which a pivotal transformation unfolds marking the stability switching of a two-step nucleation process, unveiling a previously unnoticed mid-density state bridging high- and low-density liquid states. These findings redefine our understanding of liquid-liquid phase transition, contributing to a comprehensive phase diagram for supercooled water including the elusive "no man’s land, unravelling its intricate complexity.