Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
J Chem Phys. 2017 Apr 21;146(15):154109. doi: 10.1063/1.4981009.
Linear response approximations are central to our understanding and simulations of nonequilibrium statistical mechanics. Despite the success of these approaches in predicting nonequilibrium dynamics, open questions remain. Laird and Thompson [J. Chem. Phys. 126, 211104 (2007)] previously formalized, in the context of solvation dynamics, the connection between the static linear-response approximation and the assumption of Gaussian statistics. The Gaussian statistics perspective is useful in understanding why linear response approximations are still accurate for perturbations much larger than thermal energies. In this paper, we use this approach to address three outstanding issues in the context of the "dipole-flip" model, which is known to exhibit nonlinear response. First, we demonstrate how non-Gaussian statistics can be predicted from purely equilibrium molecular dynamics (MD) simulations (i.e., without resort to a full nonequilibrium MD as is the current practice). Second, we show that the Gaussian statistics approximation may also be used to identify the physical origins of nonlinear response residing in a small number of coordinates. Third, we explore an approach for correcting the Gaussian statistics approximation for nonlinear response effects using the same equilibrium simulation. The results are discussed in the context of several other examples of nonlinear responses throughout the literature.
线性响应近似是我们理解和模拟非平衡统计力学的核心。尽管这些方法在预测非平衡动力学方面取得了成功，但仍存在一些悬而未决的问题。Laird 和 Thompson [J. Chem. Phys. 126, 211104 (2007)] 之前在溶剂化动力学的背景下，将静态线性响应近似与高斯统计假设之间的联系形式化。高斯统计观点有助于理解为什么线性响应近似在比热能量大得多的微扰下仍然准确。在本文中，我们使用这种方法来解决“偶极翻转”模型背景下三个突出的问题，该模型已知表现出非线性响应。首先，我们展示了如何仅通过平衡分子动力学（MD）模拟（即，不采用当前实践中完全非平衡 MD 的方法）来预测非高斯统计。其次，我们表明，高斯统计近似也可用于识别非线性响应中位于少数坐标的物理起源。第三，我们探索了一种使用相同平衡模拟校正非线性响应效应的高斯统计近似的方法。结果在文献中几个其他非线性响应示例的背景下进行了讨论。
