Institute for Theoretical Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany.
J Chem Phys. 2017 Feb 21;146(7):074105. doi: 10.1063/1.4976129.
Microcanonical instanton theory offers the promise of providing rate constants for chemical reactions including quantum tunneling of atoms over the whole temperature range. We discuss different rate expressions, which require the calculation of stability parameters of the instantons. The traditional way of obtaining these stability parameters is shown to be numerically unstable in practical applications. We provide three alternative algorithms to obtain such stability parameters for non-separable systems, i.e., systems in which the vibrational modes perpendicular to the instanton path couple to movement along the path. We show the applicability of our algorithms on two molecular systems: H + OH → HO + H using a fitted potential energy surface and HNCO + H → NHCO using a potential obtained on-the-fly from density functional calculations.
微正则系综瞬时理论有望为化学反应提供速率常数，包括整个温度范围内原子的量子隧穿。我们讨论了不同的速率表达式，这些表达式需要计算瞬时的稳定性参数。传统的获得这些稳定性参数的方法在实际应用中被证明是数值不稳定的。我们提供了三种替代算法，用于获得非可分离系统（即垂直于瞬时路径的振动模式与沿路径的运动耦合的系统）的稳定性参数。我们在两个分子系统上展示了我们算法的适用性：使用拟合势能面的 H + OH → HO + H 和使用从密度泛函计算中实时获得的势能的 HNCO + H → NHCO。
