Department of Chemistry, Lomonosov Moscow State University, Leninskiye Gory 1-3, Moscow 119991, Russian Federation.
Department of Chemistry, Rice University, Houston, Texas 77005, USA.
J Chem Phys. 2018 Feb 28;148(8):084102. doi: 10.1063/1.5020295.
Three dihalogenated methane derivatives (CHF, CHFCl, and CHCl) were used as model systems to compare and assess the accuracy of two different approaches for predicting observed fundamental frequencies: canonical operator Van Vleck vibrational perturbation theory (CVPT) and vibrational configuration interaction (VCI). For convenience and consistency, both methods employ the Watson Hamiltonian in rectilinear normal coordinates, expanding the potential energy surface (PES) as a Taylor series about equilibrium and constructing the wavefunction from a harmonic oscillator product basis. At the highest levels of theory considered here, fourth-order CVPT and VCI in a harmonic oscillator basis with up to 10 quanta of vibrational excitation in conjunction with a 4-mode representation sextic force field (SFF-4MR) computed at MP2/cc-pVTZ with replacement CCSD(T)/aug-cc-pVQZ harmonic force constants, the agreement between computed fundamentals is closer to 0.3 cm on average, with a maximum difference of 1.7 cm. The major remaining accuracy-limiting factors are the accuracy of the underlying electronic structure model, followed by the incompleteness of the PES expansion. Nonetheless, computed and experimental fundamentals agree to within 5 cm, with an average difference of 2 cm, confirming the utility and accuracy of both theoretical models. One exception to this rule is the formally IR-inactive but weakly allowed through Coriolis-coupling H-C-H out-of-plane twisting mode of dichloromethane, whose spectrum we therefore revisit and reassign. We also investigate convergence with respect to order of CVPT, VCI excitation level, and order of PES expansion, concluding that premature truncation substantially decreases accuracy, although VCI(6)/SFF-4MR results are still of acceptable accuracy, and some error cancellation is observed with CVPT2 using a quartic force field.
三种二卤代甲烷衍生物（CHF、CHFCl 和 CHCl）被用作模型系统，以比较和评估两种不同方法预测观测到的基本频率的准确性：正则算符范维克拉克振动微扰理论（CVPT）和振动组态相互作用（VCI）。为了方便和一致性，两种方法都在直线正则坐标中采用 Watson 哈密顿量，将势能面（PES）展开为平衡处的泰勒级数，并从谐振子乘积基构建波函数。在本文考虑的最高理论水平上，四阶 CVPT 和 VCI 在谐振子基中，最高激发 10 个振动量子，结合四模式表示 sextic 力场（SFF-4MR），在 MP2/cc-pVTZ 上计算，用替换 CCSD(T)/aug-cc-pVQZ 谐振力常数，计算出的基本频率之间的一致性平均更接近 0.3cm，最大差异为 1.7cm。主要的剩余精度限制因素是基础电子结构模型的准确性，其次是 PES 展开的不完整性。尽管如此，计算和实验基本频率的一致性在 5cm 以内，平均差异为 2cm，这证实了两种理论模型的实用性和准确性。有一个例外，即二氯甲烷的形式上 IR 非活性但通过科里奥利耦合 H-C-H 面外扭曲模式弱允许，因此我们重新考察并重新分配其光谱。我们还研究了 CVPT 阶数、VCI 激发水平和 PES 展开阶数的收敛性，得出过早截断会大大降低精度的结论，尽管 VCI(6)/SFF-4MR 的结果仍然具有可接受的精度，并且使用四次力场的 CVPT2 观察到一些误差抵消。
