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Comparing the accuracy of perturbative and variational calculations for predicting fundamental vibrational frequencies of dihalomethanes

机译：比较扰动和变分计算的准确性，以预测二卤代甲烷的基本振动频率

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摘要

Three dihalogenated methane derivatives (CH2F2, CH2FCl, and CH2Cl2) 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(-1) on average, with a maximum difference of 1.7 cm(-1). 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(-1), with an average difference of 2 cm(-1), 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-Hout-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. Published by AIP Publishing.
机译：使用三种二卤化甲烷衍生物（CH2F2，CH2FCl和CH 2 Cl 2）作为模型系统，以比较和评估预测观察到的基本频率的两种不同方法的准确性：规范操作员范vleck振动扰动理论（CVPT）和振动配置相互作用（VCI） 。为方便和一致性，这两种方法都采用了Watson Hamiltonian在直线正常坐标，将潜在的能量表面（PE）扩展为泰勒序列关于平衡和构建谐波振荡产品的基础。在这里考虑的最高级别的理论，四阶CVPT和VCI在谐振子基础上，与在MP2 / CC-计算的4模式表示奇力力字段（SFF-4MR）结合多达10个振动激发。 PVTZ具有替代CCSD（T）/ Aug-CC-PVQZ谐波常数，计算基本面之间的协议平均更接近0.3厘米（-1），最大差为1.7厘米（-1）。主要的剩余精度限制因素是底层电子结构模型的准确性，其次是对PES扩张的不完整性。尽管如此，计算和实验基础达到5厘米（-1）范围内，平均差异为2厘米（-1），确认了理论模型的效用和准确性。该规则的一个例外是正式的IR-非活性，但是通过科里奥利耦合H-C-Hout-平面扭曲模式的二氯甲烷的频谱，因此我们因此重新分配和重新分配。我们还调查了对CVPT的顺序的收敛，VCI激励水平和PES扩展的顺序，结论是过早截断大幅度降低的准确性，尽管VCI（6）/ SFF-4MR结果仍然是可接受的准确性，但取消一些错误取消使用四分之一力场与CVPT2观察。通过AIP发布发布。

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