An analysis of fluctuations in supercooled TIP4P/2005 water.
An analysis of fluctuations in supercooled TIP4P/2005 water.
复制标题
过冷 TIP4P/2005 水的波动分析。
作者：
S. Overduin;G. Patey
采用大尺度、长时间的分子动力学模拟研究了TIP4P/2005水模型在过冷区（240-190 K）的波动。特别注意集中在附近的一个先前报道的液液临界点[J]。L. F. Abascal和C. Vega， J. Chem。[j].物理学报，2016,33(5):555 - 557。水被看作是一种等摩尔二元混合物，其“种类”是根据局部四面体顺序参数来定义的。batia - thornton涨落分析表明，物种浓度涨落与密度涨落耦合，完全解释了过冷条件下小波数结构因子的异常增加。虽然我们发现沿提出的临界等差线浓度和密度波动随温度的降低而增加，但我们不能证实液-液临界点的存在。我们的模拟表明，先前报道的参数不是一个真正的液-液临界点，我们没有发现两相共存的证据在其附近。结果表明，要获得深度过冷水的密度分布，需要很长的模拟时间（500个分子8 μs左右），这在寻找两相区存在的直接证据时尤为重要。
Large-scale, long-time molecular dynamics simulations are used to investigate fluctuations in the TIP4P/2005 water model in the supercooled region (240-190 K). Particular attention is focused in the vicinity of a previously reported liquid-liquid critical point [J. L. F. Abascal and C. Vega, J. Chem. Phys. 133, 234502 (2010)]. Water is viewed as an equimolar binary mixture with "species" defined based on a local tetrahedral order parameter. A Bhatia-Thornton fluctuation analysis is used to show that species concentration fluctuations couple to density fluctuations and completely account for the anomalous increase in the structure factor at small wave number observed under supercooled conditions. Although we find that both concentration and density fluctuations increase with decreasing temperature along the proposed critical isochore, we cannot confirm the existence of a liquid-liquid critical point. Our simulations suggest that the parameters previously reported are not a true liquid-liquid critical point and we find no evidence of two-phase coexistence in its vicinity. It is shown that very long simulations (on the order of 8 μs for 500 molecules) are necessary to obtain well converged density distributions for deeply supercooled water and this is especially important if one is seeking direct evidence of a two-phase region.