Post-CCSD(T) corrections to bond distances and vibrational frequencies: the power of Λ

The importance of post-CCSD(T) corrections as high as CCSDTQ56 for ground-state spectroscopic constants (De, ωe, ωexe, and αe) has been surveyed for a sample of two dozen mostly heavy-atom diatomics spanning a broad range of static correlation strength. While CCSD(T) is known to be an unusually felicitous ‘Pauling point’ between accuracy and computational cost, performance leaves something to be desired for molecules with strong static correlation. We find CCSDT(Q)Λ to be the next ‘sweet spot’ up, of comparable or superior quality to the much more expensive CCSDTQ. A similar comparison applies to CCSDTQ(5)Λ vs. CCSDTQ5, while CCSDTQ5(6)Λ is essentially indistinguishable from CCSDTQ56. A composite of CCSD(T)-X2C/ACV5Z-X2C with [CCSDT(Q)Λ – CCSD(T)]/cc-pVTZ or even cc-pVDZ basis sets appears highly effective for computational vibrational spectroscopy. Unlike CCSDT(Q) which breaks down for the ozone vibrational frequencies, CCSDT(Q)Λ handles them gracefully.

本研究针对24种以重原子双原子分子为主、覆盖宽泛静态关联强度范围的样本，系统调研了最高至CCSDTQ56的后CCSD(T)校正（post-CCSD(T) corrections）对基态光谱常数（De、ωe、ωexe及αe）的重要性。尽管耦合簇单双激发加微扰三重激发（CCSD(T)）因在计算精度与成本间达成了罕见的理想平衡，被称为难得的“鲍林点（Pauling point）”，但对于强静态关联的分子，其计算表现仍有待改进。本研究发现，CCSDT(Q)Λ是下一阶最优平衡点，其计算精度可与成本高昂得多的CCSDTQ相媲美，甚至更优。CCSDTQ(5)Λ与CCSDTQ5之间也存在类似的对比结论，而CCSDTQ5(6)Λ与CCSDTQ56的计算结果几乎无差异。采用CCSD(T)-X2C/ACV5Z-X2C结合[CCSDT(Q)Λ - CCSD(T)]/cc-pVTZ乃至cc-pVDZ基组的复合计算方案，在计算振动光谱方面展现出极佳的效果。与在臭氧振动频率计算中失效的CCSDT(Q)不同，CCSDT(Q)Λ可以稳健且顺利地完成此类计算。