Departamento de Física, Universidade Federal de Santa Catarina, Florianópolis, SC, 88010-970, Brazil.
J Chem Phys. 2011 Jul 28;135(4):044517. doi: 10.1063/1.3613669.
Molecular dynamics simulations are used to examine the relationship between water-like anomalies and the liquid-liquid critical point in a family of model fluids with multi-Gaussian, core-softened pair interactions. The core-softened pair interactions have two length scales, such that the longer length scale associated with a shallow, attractive well is kept constant while the shorter length scale associated with the repulsive shoulder is varied from an inflection point to a minimum of progressively increasing depth. The maximum depth of the shoulder well is chosen so that the resulting potential reproduces the oxygen-oxygen radial distribution function of the ST4 model of water. As the shoulder well depth increases, the pressure required to form the high density liquid decreases and the temperature up to which the high-density liquid is stable increases, resulting in the shift of the liquid-liquid critical point to much lower pressures and higher temperatures. To understand the entropic effects associated with the changes in the interaction potential, the pair correlation entropy is computed to show that the excess entropy anomaly diminishes when the shoulder well depth increases. Excess entropy scaling of diffusivity in this class of fluids is demonstrated, showing that decreasing strength of the excess entropy anomaly with increasing shoulder depth results in the progressive loss of water-like thermodynamic, structural and transport anomalies. Instantaneous normal mode analysis was used to index the overall curvature distribution of the fluid and the fraction of imaginary frequency modes was shown to correlate well with the anomalous behavior of the diffusivity and the pair correlation entropy. The results suggest in the case of core-softened potentials, in addition to the presence of two length scales, energetic, and entropic effects associated with local minima and curvatures of the pair interaction play an important role in determining the presence of water-like anomalies and the liquid-liquid phase transition.
分子动力学模拟用于研究多高斯核心软化对相互作用模型流体中与类水异常和液-液相临界点之间的关系。核心软化对相互作用具有两个长度尺度，使得与浅吸引阱相关的较长长度尺度保持不变，而与排斥肩部相关的较短长度尺度从转折点到逐渐增加深度的最小值变化。肩部阱的最大深度选择使得所得到的势再现水的 ST4 模型的氧-氧径向分布函数。随着肩部阱深度的增加，形成高密度液体所需的压力降低，高密度液体稳定的温度升高，导致液-液相临界点转移到更低的压力和更高的温度。为了理解与相互作用势变化相关的熵效应，计算了对关联熵以表明当肩部阱深度增加时过剩熵异常减小。证明了在这类流体中扩散率的过剩熵标度，表明随着肩部深度的增加，过剩熵异常的强度减小导致类水热力学、结构和输运异常的逐渐丧失。瞬时正常模式分析用于指数化流体的整体曲率分布，并且虚频模式的分数与扩散率和对关联熵的异常行为很好地相关。结果表明，在核心软化势的情况下，除了存在两个长度尺度之外，与局部最小值和对相互作用曲率相关的能量和熵效应在确定类水异常和液-液相相变的存在方面起着重要作用。
