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....
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