Eltareb Ali, Lopez Gustavo E, Giovambattista Nicolas
Department of Physics, Brooklyn College of the City University of New York, Brooklyn, New York 11210 USA.
Ph.D. Program in Physics, The Graduate Center of the City University of New York, New York, NY 10016 USA.
Sci Rep. 2022 Apr 9;12(1):6004. doi: 10.1038/s41598-022-09525-x.
We perform path-integral molecular dynamics (PIMD), ring-polymer MD (RPMD), and classical MD simulations of H[Formula: see text]O and D[Formula: see text]O using the q-TIP4P/F water model over a wide range of temperatures and pressures. The density [Formula: see text], isothermal compressibility [Formula: see text], and self-diffusion coefficients D(T) of H[Formula: see text]O and D[Formula: see text]O are in excellent agreement with available experimental data; the isobaric heat capacity [Formula: see text] obtained from PIMD and MD simulations agree qualitatively well with the experiments. Some of these thermodynamic properties exhibit anomalous maxima upon isobaric cooling, consistent with recent experiments and with the possibility that H[Formula: see text]O and D[Formula: see text]O exhibit a liquid-liquid critical point (LLCP) at low temperatures and positive pressures. The data from PIMD/MD for H[Formula: see text]O and D[Formula: see text]O can be fitted remarkably well using the Two-State-Equation-of-State (TSEOS). Using the TSEOS, we estimate that the LLCP for q-TIP4P/F H[Formula: see text]O, from PIMD simulations, is located at [Formula: see text] MPa, [Formula: see text] K, and [Formula: see text] g/cm[Formula: see text]. Isotope substitution effects are important; the LLCP location in q-TIP4P/F D[Formula: see text]O is estimated to be [Formula: see text] MPa, [Formula: see text] K, and [Formula: see text] g/cm[Formula: see text]. Interestingly, for the water model studied, differences in the LLCP location from PIMD and MD simulations suggest that nuclear quantum effects (i.e., atoms delocalization) play an important role in the thermodynamics of water around the LLCP (from the MD simulations of q-TIP4P/F water, [Formula: see text] MPa, [Formula: see text] K, and [Formula: see text] g/cm[Formula: see text]). Overall, our results strongly support the LLPT scenario to explain water anomalous behavior, independently of the fundamental differences between classical MD and PIMD techniques. The reported values of [Formula: see text] for D[Formula: see text]O and, particularly, H[Formula: see text]O suggest that improved water models are needed for the study of supercooled water.
我们使用q-TIP4P/F水模型在广泛的温度和压力范围内对H₂O和D₂O进行路径积分分子动力学（PIMD）、环聚合物分子动力学（RPMD）以及经典分子动力学模拟。H₂O和D₂O的密度ρ、等温压缩率κ以及自扩散系数D(T)与现有的实验数据高度吻合；从PIMD和分子动力学模拟得到的等压热容CP与实验结果在定性上吻合良好。其中一些热力学性质在等压冷却时呈现出异常的最大值，这与最近的实验结果一致，也与H₂O和D₂O在低温和正压下可能存在液-液临界点（LLCP）的情况相符。H₂O和D₂O的PIMD/分子动力学数据可以使用双态状态方程（TSEOS）进行非常好的拟合。使用TSEOS，我们估计从PIMD模拟得到的q-TIP4P/F H₂O的LLCP位于216.5 MPa、236.1 K和0.934 g/cm³。同位素取代效应很重要；q-TIP4P/F D₂O中LLCP的位置估计为217.2 MPa、235.3 K和0.937 g/cm³。有趣的是，对于所研究的水模型，PIMD和分子动力学模拟在LLCP位置上的差异表明核量子效应（即原子离域）在LLCP附近水的热力学中起着重要作用（从q-TIP4P/F水的分子动力学模拟来看，为221.4 MPa、233.0 K和0.940 g/cm³）。总体而言，我们的结果有力地支持了液态液态相变理论（LLPT）来解释水的异常行为，而与经典分子动力学和PIMD技术之间的根本差异无关。所报道的D₂O以及特别是H₂O的CP值表明，对于过冷水的研究需要改进水模型。
