Tao Xuecheng, Shushkov Philip, Miller Thomas F
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
J Chem Phys. 2020 Mar 31;152(12):124117. doi: 10.1063/1.5144307.
We address the calculation of microcanonical reaction rates for processes involving significant nuclear quantum effects using ring-polymer molecular dynamics (RPMD), both with and without electronically non-adiabatic transitions. After illustrating the shortcomings of the naive free-particle direct-shooting method, in which the temperature of the internal ring-polymer modes is set to the translational energy scale, we investigate alternative strategies based on the expression for the microcanonical rate in terms of the inverse Laplace transform of the thermal reaction rate. It is shown that simple application of the stationary-phase approximation (SPA) dramatically improves the performance of the microcanonical rates using RPMD, particularly in the low-energy region where tunneling dominates. Using the SPA as a Bayesian prior, numerically exact RPMD microcanonical rates are then obtained using maximum entropy inversion of the thermal reaction rates for both electronically adiabatic and non-adiabatic model systems. Finally, the direct-shooting method is revisited using the SPA-determined temperature for the internal ring-polymer modes, leading to a simple, direct-simulation method with improved accuracy in the tunneling regime. This work suggests a general strategy for the extraction of microcanonical dynamical quantities from RPMD (or other approximate thermal) simulations.
我们使用环聚合物分子动力学（RPMD）来处理涉及显著核量子效应的过程的微正则反应速率计算，包括有无电子非绝热跃迁的情况。在说明了将内部环聚合物模式的温度设定为平移能量尺度的朴素自由粒子直接射击方法的缺点之后，我们基于微正则速率关于热反应速率的逆拉普拉斯变换的表达式研究了替代策略。结果表明，平稳相位近似（SPA）的简单应用显著提高了使用RPMD计算微正则速率的性能，特别是在隧穿占主导的低能量区域。将SPA用作贝叶斯先验，然后通过对电子绝热和非绝热模型系统的热反应速率进行最大熵反演，获得了数值精确的RPMD微正则速率。最后，使用SPA确定的内部环聚合物模式温度重新审视直接射击方法，从而得到一种在隧穿区域具有更高精度的简单直接模拟方法。这项工作提出了一种从RPMD（或其他近似热）模拟中提取微正则动力学量的通用策略。
