擦亮黄金标准：轨道选择在CCSD（T）振动频率预测中的作用
Polishing the Gold Standard: The Role of Orbital Choice in CCSD(T) Vibrational Frequency Prediction
摘要：
虽然具有自旋限制的Hartree-Fock（RHF）轨道的CCSD（T）长期以来因其准确描述闭壳层相互作用的能力而受到称赞，但CCSD（T）对开壳层物质的性能更加不稳定，特别是当使用自旋不受限制的HF（UHF）参考时。以前的研究表明，当使用非HF分子轨道集（如Brueckner或Kohn-Sham密度泛函理论（DFT）轨道）作为参考时，开壳层系统的处理得到了改进。受正则化轨道优化的二阶Møller-Plesset微扰理论的成功启发（$κ$ -OOMP 2）轨道作为MP3的参考轨道，我们研究了使用$κ$ -OOMP 2轨道和各种DFT轨道作为参考轨道，用于CCSD（T）计算一组36个闭壳层的修正基态谐振频率（29个中性粒子，6个阳离子，1个阴离子）和59个开壳层双原子物种（38个中性粒子，15个阳离子，6个阴离子）。在这种情况下，使用$κ$ -OOMP 2轨道简化了观察到的UHF轨道和OOMP 2轨道的困难情况。去除2个多参考系统和12个具有模糊实验数据的系统，留下修剪的数据集。在修剪数据集上的总体性能突出显示了CCSD（T）与B 97轨道参考（CCSD（T）：B 97）、CCSD（T）与$κ$ -OOMP 2轨道参考（CCSD（T）：$κ$ -OOMP 2）和CCSD（T）与B 97 M-rV轨道参考（CCSD（T）：B 97 M-rV），均方根值分别为8.48 cm $^{-1}$和8.50 cm $^{-1}$，以及8.75 cm $^{-1}$，优于CCSD（T）：UHF近5倍。此外，闭壳和开壳子集上的性能表明，这些方法能够以相当的精度和鲁棒性处理开壳和闭壳系统。$κ$ -OOMP 2轨道的使用也被证明在诊断可能妨碍CCSD（T）可靠性的多参考特征方面是有用的。
英文摘要：
While CCSD(T) with spin-restricted Hartree-Fock (RHF) orbitals has long been lauded for its ability to accurately describe closed-shell interactions, the performance of CCSD(T) on open-shell species is much more erratic, especially when using a spin-unrestricted HF (UHF) reference. Previous studies have shown improved treatment of open-shell systems when a non-HF set of molecular orbitals, like Brueckner or Kohn-Sham density functional theory (DFT) orbitals, is used as a reference. Inspired by the success of regularized orbital-optimized second-order Møller-Plesset perturbation theory ($κ$-OOMP2) orbitals as reference orbitals for MP3, we investigate the use of $κ$-OOMP2 orbitals and various DFT orbitals as reference orbitals for CCSD(T) calculations of the corrected ground-state harmonic vibrational frequencies of a set of 36 closed-shell (29 neutrals, 6 cations, 1 anion) and 59 open-shell diatomic species (38 neutrals, 15 cations, 6 anions). The use of $κ$-OOMP2 orbitals in this context alleviates difficult cases observed for both UHF orbitals and OOMP2 orbitals. Removing 2 multireference systems and 12 systems with ambiguous experimental data leaves a pruned data set. Overall performance on the pruned data set highlights CCSD(T) with a B97 orbital reference (CCSD(T):B97), CCSD(T) with a $κ$-OOMP2 orbital reference (CCSD(T):$κ$-OOMP2), and CCSD(T) with a B97M-rV orbital reference (CCSD(T):B97M-rV) with RMSDs of 8.48 cm$^{-1}$ and 8.50 cm$^{-1}$, and 8.75 cm$^{-1}$ respectively, outperforming CCSD(T):UHF by nearly a factor of 5. Moreover, the performance on the closed- and open-shell subsets show these methods are able to treat open-shell and closed-shell systems with comparable accuracy and robustness. The use of $κ$-OOMP2 orbitals has also proven useful in diagnosing multireference character that can hinder the reliability of CCSD(T).
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