Piaggi Pablo M, Gartner Thomas E, Car Roberto, Debenedetti Pablo G
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA.
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30318, USA.
J Chem Phys. 2023 Aug 7;159(5). doi: 10.1063/5.0159288.
The possible existence of a liquid-liquid critical point in deeply supercooled water has been a subject of debate due to the challenges associated with providing definitive experimental evidence. The pioneering work by Mishima and Stanley [Nature 392, 164-168 (1998)] sought to shed light on this problem by studying the melting curves of different ice polymorphs and their metastable continuation in the vicinity of the expected liquid-liquid transition and its associated critical point. Based on the continuous or discontinuous changes in the slope of the melting curves, Mishima [Phys. Rev. Lett. 85, 334 (2000)] suggested that the liquid-liquid critical point lies between the melting curves of ice III and ice V. We explore this conjecture using molecular dynamics simulations with a machine learning model based on ab initio quantum-mechanical calculations. We study the melting curves of ices III, IV, V, VI, and XIII and find that all of them are supercritical and do not intersect the liquid-liquid transition locus. We also find a pronounced, yet continuous, change in the slope of the melting lines upon crossing of the liquid locus of maximum compressibility. Finally, we analyze the literature in light of our findings and conclude that the scenario in which the melting curves are supercritical is favored by the most recent computational and experimental evidence. Although the preponderance of evidence is consistent with the existence of a second critical point in water, the behavior of ice polymorph melting lines does not provide strong evidence in support of this viewpoint, according to our calculations.
由于难以提供确凿的实验证据，深度过冷水体系中可能存在液-液临界点这一问题一直备受争议。Mishima和Stanley [《自然》392, 164 - 168 (1998)] 的开创性工作试图通过研究不同冰多晶型物的熔化曲线及其在预期液-液转变及其相关临界点附近的亚稳延伸来阐明这一问题。基于熔化曲线斜率的连续或不连续变化，Mishima [《物理评论快报》85, 334 (2000)] 认为液-液临界点位于冰III和冰V的熔化曲线之间。我们使用基于第一性原理量子力学计算的机器学习模型，通过分子动力学模拟来探究这一推测。我们研究了冰III、IV、V、VI和XIII的熔化曲线，发现它们都是超临界的，并且不与液-液转变轨迹相交。我们还发现在穿过最大压缩性液体轨迹时，熔化线的斜率会发生明显但连续的变化。最后，我们根据研究结果分析了相关文献，并得出结论：熔化曲线为超临界的情况得到了最新计算和实验证据的支持。尽管大量证据与水中存在第二个临界点的观点一致，但根据我们的计算，冰多晶型熔化线的行为并不能为这一观点提供有力证据。
