(252e) Long-Range Structural Features in the Amorphous Ices Could be a Signature of Criticality in Supercooled Liquid Water

There is longstanding debate about the relationship between polyamorphism in glassy water and the possibility of a metastable liquid-liquid phase transition (and a corresponding liquid-liquid critical point, LLCP) in supercooled liquid water. Despite the natural analogies between the low-density and high-density forms of amorphous ice (LDA and HDA, respectively) and the low-density and high-density forms of supercooled liquid water (LDL and HDL, respectively), it is unclear whether the experimentally observed LDA and HDA structures necessarily constitute proof of the existence of LDL, HDL, or an LLCP in real water. Motivated by the recent finding that long-range structural metrics (i.e., long-wavelength density fluctuations characterized by the low-wavenumber limit of the structure factor S(k)) can distinguish the pressure-induced transition between LDA and HDA in molecular simulations [F. Martelli et al., Phys. Rev. Lett. 119, 136002 (2017)], we perform extensive nonequilibrium molecular dynamics simulations of isobaric cooling trajectories at various pressures, using the TIP4P/2005 and mW water models. Starting from the liquid state, we track the long-range structure of water as a function of temperature and pressure, which reveals that models that possess an LLCP (TIP4P/2005) exhibit a non-monotonic trend in long-range density fluctuations as a function of pressure; these density fluctuations peak near the critical pressure and persist even as the system is cooled into the glassy state. Conversely, models that do not exhibit an LLCP (mW) exhibit a purely monotonic dependence of density fluctuations with pressure. These findings could indicate a structural metric to detect whether a system exhibits an LLCP, and may shed light on the relationship between water's thermophysical anomalies and the existence of a metastable LLCP.