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What Is a Simple Liquid?

机译：什么是简单液体？

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This paper is an attempt to identify the real essence of simplicity of liquids in John Locke’s understanding of the term. Simple liquids are traditionally defined as many-body systems of classical particles interacting via radially symmetric pair potentials. We suggest that a simple liquid should be defined instead by the property of having strong correlations between virial and potential-energy equilibrium fluctuations in the NVT ensemble. There is considerable overlap between the two definitions, but also some notable differences. For instance, in the new definition simplicity is not a direct property of the intermolecular potential because a liquid is usually only strongly correlating in part of its phase diagram. Moreover, not all simple liquids are atomic (i.e., with radially symmetric pair potentials) and not all atomic liquids are simple. The main part of the paper motivates the new definition of liquid simplicity by presenting evidence that a liquid is strongly correlating if and only if its intermolecular interactions may be ignored beyond the first coordination shell (FCS). This is demonstrated by NVT simulations of the structure and dynamics of several atomic and three molecular model liquids with a shifted-forces cutoff placed at the first minimum of the radial distribution function. The liquids studied are inverse power-law systems (r-n pair potentials with n=18,6,4), Lennard-Jones (LJ) models (the standard LJ model, two generalized Kob-Andersen binary LJ mixtures, and the Wahnstrom binary LJ mixture), the Buckingham model, the Dzugutov model, the LJ Gaussian model, the Gaussian core model, the Hansen-McDonald molten salt model, the Lewis-Wahnstrom ortho-terphenyl model, the asymmetric dumbbell model, and the single-point charge water model. The final part of the paper summarizes properties of strongly correlating liquids, emphasizing that these are simpler than liquids in general. Simple liquids, as defined here, may be characterized in three quite different ways: (1)?chemically by the fact that the liquid’s properties are fully determined by interactions from the molecules within the FCS, (2)?physically by the fact that there are isomorphs in the phase diagram, i.e., curves along which several properties like excess entropy, structure, and dynamics, are invariant in reduced units, and (3)?mathematically by the fact that throughout the phase diagram the reduced-coordinate constant-potential-energy hypersurfaces define a one-parameter family of compact Riemannian manifolds. No proof is given that the chemical characterization follows from the strong correlation property, but we show that this FCS characterization is consistent with the existence of isomorphs in strongly correlating liquids’ phase diagram. Finally, we note that the FCS characterization of simple liquids calls into question the physical basis of standard perturbation theory, according to which the repulsive and attractive forces play fundamentally different roles for the physics of liquids.
机译：本文旨在通过约翰·洛克（John Locke）对该术语的理解来确定液体简单性的真正实质。传统上，简单液体定义为经典粒子的多体系统通过径向对称的对电势相互作用。我们建议应使用NVT集合中的病毒和势能平衡波动之间具有很强相关性的属性来定义一种简单的液体。两种定义之间存在相当大的重叠，但也存在一些显着差异。例如，在新定义中，简单性不是分子间电势的直接属性，因为液体通常仅在其相图的一部分中具有很强的相关性。此外，并非所有简单的液体都是原子的（即，具有径向对称的对电位），也不是所有的原子液体都是简单的。本文的主要部分通过提供证据表明，当且仅当其分子间的相互作用在第一配位壳（FCS）之外可以忽略时，液体才具有强关联性，从而激发了对液体简单性的新定义。 NVT对几种原子和三种分子模型液体的结构和动力学进行的NVT模拟证明了这一点，其位移力截止值位于径向分布函数的第一个最小值。研究的液体是逆幂律系统（n对电势，n = 18,6,4），Lennard-Jones（LJ）模型（标准LJ模型，两种广义的Kob-Andersen二元LJ混合物和Wahnstrom二元LJ）混合），白金汉模型，Dzugutov模型，LJ高斯模型，高斯岩心模型，Hansen-McDonald熔融盐模型，Lewis-Wahnstrom邻三联苯模型，不对称哑铃模型和单点电荷水模型。本文的最后一部分总结了强关联液体的性质，强调这些液体通常比液体更简单。此处定义的简单液体可以通过三种截然不同的方式来表征：（1）化学上，液体的性质完全由FCS中分子的相互作用决定，（2）物理上，是相图中的同构，即曲线，沿着这些曲线，多余的熵，结构和动力学等一些属性在缩减的单位中是不变的;以及（3）在数学上是由于在整个相图中还原坐标为恒势能量超曲面定义了紧凑的黎曼流形的一参数族。没有证据表明化学特征是由强相关性引起的，但是我们证明了该FCS的特征与强相关液体相图中同晶型的存在是一致的。最后，我们注意到，简单液体的FCS表征使标准扰动理论的物理基础受到质疑，据此，排斥力和吸引力在液体物理学中起着根本不同的作用。

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