State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, Frontier Science Center for Nano-optoelectronics and School of Physics, Peking University, Beijing 100871, People's Republic of China.
Interdisciplinary Institute of Light-Element Quantum Materials, Research Center for Light-Element Advanced Materials, and Collaborative Innovation Center of Quantum Materials, Peking University, Beijing 100871, People's Republic of China.
J Chem Phys. 2023 Jun 14;158(22). doi: 10.1063/5.0146562.
Tunneling splittings observed in molecular rovibrational spectra are significant evidence for tunneling motion of hydrogen nuclei in water clusters. Accurate calculations of the splitting sizes from first principles require a combination of high-quality inter-atomic interactions and rigorous methods to treat the nuclei with quantum mechanics. Many theoretical efforts have been made in recent decades. This Perspective focuses on two path-integral based tunneling splitting methods whose computational cost scales well with the system size, namely, the ring-polymer instanton method and the path-integral molecular dynamics (PIMD) method. From a simple derivation, we show that the former is a semiclassical approximation to the latter, despite that the two methods are derived very differently. Currently, the PIMD method is considered to be an ideal route to rigorously compute the ground-state tunneling splitting, while the instanton method sacrifices some accuracy for a significantly smaller computational cost. An application scenario of such a quantitatively rigorous calculation is to test and calibrate the potential energy surfaces of molecular systems by spectroscopic accuracy. Recent progress in water clusters is reviewed, and the current challenges are discussed.
在分子振转光谱中观察到的隧道分裂是氢原子核在水分子簇中隧道运动的重要证据。从第一性原理准确计算分裂大小需要结合高质量的原子间相互作用和严格的量子力学处理原子核的方法。近几十年来，人们进行了许多理论研究。本文重点介绍了两种基于路径积分的隧道分裂方法，其计算成本与系统大小呈良好的比例关系，即环聚合物瞬时子方法和路径积分分子动力学（PIMD）方法。通过简单的推导，我们表明前者是后者的半经典近似，尽管这两种方法的推导方式非常不同。目前，PIMD 方法被认为是严格计算基态隧道分裂的理想途径，而瞬时子方法为了显著降低计算成本而牺牲了一些准确性。这种定量严格计算的一个应用场景是通过光谱精度来测试和校准分子系统的势能面。本文回顾了水分子簇的最新进展，并讨论了当前的挑战。
