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

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