Bertels Luke W, Lee Joonho, Head-Gordon Martin
Department of Chemistry, University of California, Berkeley, California 94720, United States.
Department of Chemistry, Columbia University, New York, New York 10027, United States.
J Chem Theory Comput. 2021 Feb 9;17(2):742-755. doi: 10.1021/acs.jctc.0c00746. Epub 2021 Jan 6.
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 aug-cc-pwCVTZ basis set is used for all calculations. The use of κ-OOMP2 orbitals in this context alleviates difficult cases observed for both UHF orbitals and OOMP2 orbitals. Removing two 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, and 8.50 cm, and 8.75 cm respectively, outperforming CCSD(T):UHF by nearly a factor of 5. Moreover, the performance on the closed- and open-shell subsets shows these methods are able to treat open-shell and closed-shell systems with comparable accuracy and robustness. CCSD(T) with RHF orbitals is seen to improve upon UHF for the closed-shell species, while spatial symmetry breaking in a number of restricted open-shell HF (ROHF) references leads CCSD(T) with ROHF reference orbitals to exhibit the poorest statistical performance of all methods surveyed for open-shell species. The use of κ-OOMP2 orbitals has also proven useful in diagnosing multireference character that can hinder the reliability of CCSD(T).
虽然长期以来，具有自旋限制哈特里 - 福克（RHF）轨道的耦合簇单双激发（T）近似（CCSD(T)）因其能够准确描述闭壳层相互作用的能力而备受赞誉，但CCSD(T)在开壳层物种上的表现却更加不稳定，尤其是在使用自旋非限制HF（UHF）参考时。先前的研究表明，当使用一组非HF分子轨道，如布吕克纳或科恩 - 沙姆密度泛函理论（DFT）轨道作为参考时，对开壳层系统的处理得到了改进。受正则化轨道优化二阶莫勒 - 普莱塞特微扰理论（κ - OOMP2）轨道作为MP3参考轨道成功的启发，我们研究了使用κ - OOMP2轨道和各种DFT轨道作为参考轨道，对一组36个闭壳层（29个中性分子、6个阳离子、1个阴离子）和59个开壳层双原子物种（38个中性分子、15个阳离子、6个阴离子）的校正基态谐波振动频率进行CCSD(T)计算。所有计算均使用aug - cc - pwCVTZ基组。在这种情况下使用κ - OOMP2轨道缓解了UHF轨道和OOMP2轨道所观察到的困难情况。去除两个多参考系统和12个具有模糊实验数据的系统后得到一个精简数据集。精简数据集上的总体性能突出了以B97轨道为参考的CCSD(T)（CCSD(T):B97）、以κ - OOMP2轨道为参考的CCSD(T)（CCSD(T):κ - OOMP2）和以B97M - rV轨道为参考的CCSD(T)（CCSD(T):B97M - rV），其均方根偏差（RMSD）分别为8.48 cm、8.50 cm和8.75 cm，比CCSD(T):UHF的性能高出近5倍。此外，在闭壳层和开壳层子集上的性能表明，这些方法能够以相当的准确性和稳健性处理开壳层和闭壳层系统。对于闭壳层物种，具有RHF轨道的CCSD(T)比UHF有所改进，而在一些限制开壳层HF（ROHF）参考中的空间对称性破缺导致具有ROHF参考轨道的CCSD(T)在所有调查的开壳层物种方法中表现出最差的统计性能。κ - OOMP2轨道的使用也已被证明在诊断可能阻碍CCSD(T)可靠性的多参考特征方面很有用。
