Elrod M J, Saykally R J
Department of Chemistry, University of California, Berkeley 94720, USA.
J Chem Phys. 1995 Jul 15;103(3):921-32. doi: 10.1063/1.469793.
Several commonly used approximate methods for the calculation of vibration--rotation--tunneling spectra for (HCl)2 are described. These range from one-dimensional models to an exact coupled four-dimensional treatment of the intermolecular dynamics. Two different potential surfaces were employed--an ab initio and our ES1 experimental surface (determined by imbedding the four-dimensional calculation outlined here in a least-squares loop to fit the experimental data, which is described in the accompanying paper [J. Chem. Phys. 103, 933 (1995)]. The most important conclusion deduced from this work is that the validity of the various approximate models is extremely system specific. All of the approximate methods addressed in this paper were found to be sensitive to the approximate separability of the radial and angular degrees of freedom, wherein exists the primary difference between the two potentials. Of particular importance, the commonly used reversed adiabatic angular approximation was found to be very sensitive to the choice for fixed R; an improper choice would lead to results very much different from the fully coupled results and perhaps to false conclusions concerning the intermolecular potential energy surface.
描述了几种用于计算(HCl)₂振动-转动-隧穿光谱的常用近似方法。这些方法从一维模型到分子间动力学的精确耦合四维处理。采用了两种不同的势能面——一种是从头算的，另一种是我们的ES1实验势能面（通过将此处概述的四维计算嵌入最小二乘法循环以拟合实验数据来确定，这在随附论文[J. Chem. Phys. 103, 933 (1995)]中有描述）。从这项工作得出的最重要结论是，各种近似模型的有效性极具系统特异性。本文讨论的所有近似方法都被发现对径向和角向自由度的近似可分离性敏感，而这正是两种势能之间的主要差异所在。特别重要的是，发现常用的反向绝热角近似对固定R的选择非常敏感；选择不当会导致结果与完全耦合结果大不相同，甚至可能得出关于分子间势能面的错误结论。
