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

TL;DR

This study evaluates the effectiveness of post-CCSD(T) corrections, especially those involving the $\Lambda$ method, in accurately predicting spectroscopic constants of heavy diatomic molecules, showing that certain $\Lambda$-corrected methods outperform more expensive approaches.

Contribution

It demonstrates that CCSDT(Q)$_\Lambda$ and related $\Lambda$-corrected methods provide high accuracy comparable to CCSDTQ with lower computational cost, especially for molecules with strong static correlation.

Findings

CCSDT(Q)$_\Lambda$ outperforms CCSDTQ in accuracy.

$\Lambda$-corrected methods handle ozone vibrational frequencies effectively.

Composite methods with $\Lambda$ corrections are efficient for vibrational spectroscopy.

Abstract

The importance of post-CCSD(T) corrections as high as CCSDTQ56 for ground-state spectroscopic constants ($D_e$, $\omega_e$, $\omega_ex_e$, and $\alpha_e$) 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)$_\Lambda$ 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)$_\Lambda$ vs. CCSDTQ5, while CCSDTQ5(6)$_\Lambda$ is essentially indistinguishable from CCSDTQ56. A composite of CCSD(T)-X2C/ACV5Z-X2C with [CCSDT(Q)$_\Lambda$ -- 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)$_\Lambda$ handles them gracefully.