Structural Order as the Key Phase Indicator in Supercooled Liquid Water

The phase behavior of supercooled water and the nature of its second critical point remain long-standing open questions. Using molecular dynamics simulations and enhanced sampling techniques, we demonstrate that the first-order liquid-liquid phase transition in supercooled water arises from structural reorganization between high-order and low-order liquid states, rather than density variations as traditionally assumed. Near 238K, this transition occurs with negligible density contrast, indicating that molecular geometry is the primary driver. As temperature decreases, these structural changes extend beyond the first coordination shell, and below 205K they progressively lead to more pronounced density differences. This challenges the conventional emphasis on density as the defining metric for liquid polymorphism, highlighting structural order parameters as the key indicator of phase behavior. Our findings provide a precise estimate of water’s second critical point, closely aligning with experimental observations and offering a unifying framework for understanding water’s anomalies in the deeply supercooled regime.