Levashov V A
Technological Design Institute of Scientific Instrument Engineering, Novosibirsk 630058, Russia.
J Chem Phys. 2016 Mar 7;144(9):094502. doi: 10.1063/1.4942863.
It is possible to associate with every atom or molecule in a liquid its own atomic stress tensor. These atomic stress tensors can be used to describe liquids' structures and to investigate the connection between structural and dynamic properties. In particular, atomic stresses allow to address atomic scale correlations relevant to the Green-Kubo expression for viscosity. Previously correlations between the atomic stresses of different atoms were studied using the Cartesian representation of the stress tensors or the representation based on spherical harmonics. In this paper we address structural correlations in a 3D model binary liquid using the eigenvalues and eigenvectors of the atomic stress tensors. This approach allows to interpret correlations relevant to the Green-Kubo expression for viscosity in a simple geometric way. On decrease of temperature the changes in the relevant stress correlation function between different atoms are significantly more pronounced than the changes in the pair density function. We demonstrate that this behaviour originates from the orientational correlations between the eigenvectors of the atomic stress tensors. We also found correlations between the eigenvalues of the same atomic stress tensor. For the studied system, with purely repulsive interactions between the particles, the eigenvalues of every atomic stress tensor are positive and they can be ordered: λ1 ≥ λ2 ≥ λ3 ≥ 0. We found that, for the particles of a given type, the probability distributions of the ratios (λ2/λ1) and (λ3/λ2) are essentially identical to each other in the liquids state. We also found that λ2 tends to be equal to the geometric average of λ1 and λ3. In our view, correlations between the eigenvalues may represent "the Poisson ratio effect" at the atomic scale.
可以将其自身的原子应力张量与液体中的每个原子或分子相关联。这些原子应力张量可用于描述液体的结构，并研究结构与动力学性质之间的联系。特别是，原子应力有助于处理与格林 - 库博粘度表达式相关的原子尺度相关性。此前，使用应力张量的笛卡尔表示或基于球谐函数的表示来研究不同原子的原子应力之间的相关性。在本文中，我们使用原子应力张量的特征值和特征向量来研究三维模型二元液体中的结构相关性。这种方法能够以一种简单的几何方式解释与格林 - 库博粘度表达式相关的相关性。随着温度降低，不同原子之间相关应力相关函数的变化比配对密度函数的变化明显更为显著。我们证明这种行为源于原子应力张量特征向量之间的取向相关性。我们还发现了同一原子应力张量特征值之间的相关性。对于所研究的系统，粒子之间仅存在排斥相互作用，每个原子应力张量的特征值均为正，并且可以排序为：λ1≥λ2≥λ3≥0。我们发现，对于给定类型的粒子，在液态下（λ2 / λ1）和（λ3 / λ2）比值的概率分布基本彼此相同。我们还发现λ2倾向于等于λ1和λ3的几何平均值。我们认为，特征值之间的相关性可能代表原子尺度上的“泊松比效应”。
