基于SCAN meta-GGA泛函从头算分子动力学对过冷水性质反常机制的研究

Many thermodynamical and dynamical properties of metastable supercooled water show abnormal behavior, but the mechanism is still unclear, due to the difficulty to access the supercooled region to directly investigate it by the current experiment method and the inability to accurately describe the complex interaction between atoms and molecules in computer simulations. Recently, the strong constrained and appropriately normed (SCAN) meta-GGA semilocal density functional suggested by Sun et al. [Phys. Rev. Lett. 115, 036402 (2015); Nat. Chem. 8, 831 (2016)] was proved to be able to more accurately describe the complex interaction between atoms and molecules, structural and dynamical properties of water [Proc. Natl. Acad. Sci. U.S.A. 114, 10846 (2017)]. In our previous studies, it was also found that SCAN meta-GGA can give more accurate pressure when the temperature and volume are given. These results provide guarantee for the research of water from macroscopic and microscopic aspects. In this project, the macroscopic phase behavior and the temperature- and pressure-dependent microstructure of supercooled water will be studied by ab initio molecular dynamics based on SCAN meta-GGA semilocal density functional, and the quantitative relationship between structure and abnormal properties will be investigated using appropriate order parameters. Our aim is to discover the mechanism of abnormal properties in metastable supercooled water from macroscopic and microscopic aspects. This project can not only deepen people's understanding of liquid-liquid phase transition and hydrogen bond, present microscopic explanation to some experimental results, and verify the validation of some empirical and model potentials, but also provide theoretical information for the research of other liquids showing water-like abnormal behavior.

亚稳过冷水的许多热力学和动力学性质表现出反常行为，但其机制依然不明确，原因在于目前的实验手段很难达到深过冷区进行直接研究，而计算机模拟又不能精确地描述其原子分子间复杂的相互作用。近来，Sun等建立的强约束和适当赋范（SCAN）的meta-GGA半局域密度泛函被证明能更精确地描述水中原子分子间复杂的相互作用、结构和动力学性质，我们前期的模拟研究还发现体系温度和体积一定时它能给出很准确的压强，这些结果为从宏观和微观上研究水提供了保证。本项目拟采用基于SCAN meta-GGA泛函的从头算分子动力学研究过冷水的宏观相行为和微观结构随温度和压强的变化规律，采用合适的序参数探讨结构和性质反常之间的定量关系，从宏观和微观上揭示性质反常的机制。本项目不仅能加深人们对水中液液相变和氢键的理解，对某些实验结果给出微观解释，验证某些经验势和模型势的有效性，还能为研究其它表现出似水反常行为的液体提供理论信息。

过冷水的许多热力学和动力学行为表现反常，但其机制依然不明确，原因在于实验上很难达到深过冷区进行直接研究，而模拟所需的泛函又不能精确描述其原子分子间复杂的相互作用。强约束和适当赋范（SCAN）的meta-GGA泛函已被证明能更好地描述水中原子分子间复杂的相互作用、结构和动力学性质。本项目采用基于SCAN 泛函的从头算分子动力学研究了过冷水的宏观性质和微观结构随外界条件的变化规律。研究发现：（1）基于SCAN泛函的状态方程比基于PBE的结果更接近实验值，并且在深过冷区探测到液液相变的发生；（2）在高密度水中基于SCAN泛函描述的氢键比基于PBE描述的更柔韧，更接近实验结果，而低密度水中，两种泛函描述的氢键差别不大；（3）从宏观热力学角度，虽然基于SCAN泛函在过冷水中探测到液液相变的发生，但预测的临界点倾向于与水的稳定性极限线重合，即水的相行为倾向于液液临界点缺失情形；（4）在微观上，发现水的固有结构中第二氧配位壳层的平移序有三种情况即对应着三种局域结构，结构III的比体积最大，结构II次之，结构I最小，水的密度极大值不仅来自于结构I和III之间的竞争，也来自于结构II和III之间的竞争；（5）键角分布函数的结果表明像晶态冰那样原子排列的平移序和取向序之间的耦合在水中不再存在；（6）在本项目研究的压强和温度范围内，SCAN泛函没有捕捉到动力学反常行为，原因可能在于（i）SCAN泛函本身潜在的局限性，（ii）没有考虑核量子效应，（iii）基于计算能力的限制，模拟体系原子个数少计算动力学性质误差太大。本项目基于SCAN泛函从宏观和微观两个方面阐明了液态水表现出反常热力学行为的机制，并通过对固有结构的研究加深了人们对水的结构和氢键的理解，也为非晶态冰的结构和相变研究研究提供了重要的信息。