Institute for Theoretical Chemistry , University of Stuttgart , Pfaffenwaldring 55 , 70569 Stuttgart , Germany.
J Chem Theory Comput. 2018 Apr 10;14(4):1865-1872. doi: 10.1021/acs.jctc.8b00068. Epub 2018 Mar 23.
Instanton theory is an established method to calculate rate constants of chemical reactions including atom tunneling. Technical and methodological improvements increased its applicability. Still, a large number of energy and gradient calculations is necessary to optimize the instanton tunneling path, and second derivatives of the potential energy along the tunneling path have to be evaluated, restricting the range of suitable electronic structure methods. To enhance the applicability of instanton theory, we present a dual-level approach in which instanton optimizations and Hessian calculations are performed using an efficient but approximate electronic structure method, and the potential energy along the tunneling path is recalculated using a more accurate method. This procedure extends the applicability of instanton theory to high-level electronic structure methods for which analytic gradients may not be available, like local linear-scaling approaches. We demonstrate for the analytical Eckart barrier and three molecular systems how the dual-level instanton approach corrects for the largest part of the error caused by the inaccuracy of the efficient electronic structure method. This reduces the error of the calculated rate constants significantly.
瞬子理论是一种成熟的方法，可用于计算包括原子隧穿在内的化学反应的速率常数。技术和方法上的改进提高了它的适用性。然而，优化瞬子隧穿路径需要大量的能量和梯度计算，并且必须评估势能沿隧穿路径的二阶导数，这限制了合适的电子结构方法的适用范围。为了提高瞬子理论的适用性，我们提出了一种双重水平方法，其中使用高效但近似的电子结构方法进行瞬子优化和Hessian 计算，并且使用更准确的方法重新计算隧穿路径上的势能。该程序将瞬子理论的适用性扩展到了可能没有分析梯度的高级电子结构方法，例如局部线性标度方法。我们通过分析 Eckart 势垒和三个分子系统演示了双水平瞬子方法如何纠正高效电子结构方法不准确性引起的误差的大部分。这大大降低了计算速率常数的误差。
