Nagy Tibor, Vikár Anna, Lendvay György
Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar tudósok körútja 2., H-1117 Budapest, Hungary.
J Chem Phys. 2016 Jan 7;144(1):014104. doi: 10.1063/1.4939583.
The quasiclassical trajectory (QCT) method is an efficient and important tool for studying the dynamics of bimolecular reactions. In this method, the motion of the atoms is simulated classically, and the only quantum effect considered is that the initial vibrational states of reactant molecules are semiclassically quantized. A sensible expectation is that the initial ensemble of classical molecular states generated this way should be stationary, similarly to the quantum state it is supposed to represent. The most widely used method for sampling the vibrational phase space of polyatomic molecules is based on the normal mode approximation. In the present work, it is demonstrated that normal mode sampling provides a nonstationary ensemble even for a simple molecule like methane, because real potential energy surfaces are anharmonic in the reactant domain. The consequences were investigated for reaction CH4 + H → CH3 + H2 and its various isotopologs and were found to be dramatic. Reaction probabilities and cross sections obtained from QCT calculations oscillate periodically as a function of the initial distance of the colliding partners and the excitation functions are erratic. The reason is that in the nonstationary ensemble of initial states, the mean bond length of the breaking C-H bond oscillates in time with the frequency of the symmetric stretch mode. We propose a simple method, one-period averaging, in which reactivity parameters are calculated by averaging over an entire period of the mean C-H bond length oscillation, which removes the observed artifacts and provides the physically most reasonable reaction probabilities and cross sections when the initial conditions for QCT calculations are generated by normal mode sampling.
准经典轨迹（QCT）方法是研究双分子反应动力学的一种有效且重要的工具。在该方法中，原子的运动采用经典方式模拟，唯一考虑的量子效应是反应物分子的初始振动状态进行半经典量子化。一个合理的预期是，以这种方式生成的经典分子态的初始系综应该是稳定的，类似于它所代表的量子态。对多原子分子振动相空间进行采样的最广泛使用的方法基于简正模式近似。在本工作中，证明了即使对于像甲烷这样的简单分子，简正模式采样也会提供一个非稳定系综，因为实际的势能面在反应物区域是非谐的。对反应CH4 + H → CH3 + H2及其各种同位素变体的结果进行了研究，发现结果很显著。从QCT计算得到的反应概率和截面作为碰撞伙伴初始距离的函数呈周期性振荡，并且激发函数是不规则的。原因是在初始态的非稳定系综中，断裂的C - H键的平均键长随对称伸缩模式的频率随时间振荡。我们提出了一种简单的方法，即单周期平均，其中通过对平均C - H键长振荡的整个周期进行平均来计算反应性参数，当通过简正模式采样生成QCT计算的初始条件时，该方法消除了观察到的伪影，并提供了物理上最合理的反应概率和截面。
