Can G4-like composite Ab Initio methods accurately predict vibrational harmonic frequencies?

Abstract

Minimally empirical G4-like composite wavefunction theories [E. Semidalas and J. M. L. Martin, J. Chem. Theory Comput. 16, 4238–4255 and 7507–7524 (2020)] trained against the large and chemically diverse GMTKN55 benchmark suite have demonstrated both accuracy and cost-effectiveness in predicting thermochemistry, barrier heights, and noncovalent interaction energies. Here, we assess the spectroscopic accuracy of top-performing methods: G4-n, cc-G4-n, and G4-n-F12, and validate them against explicitly correlated coupled-cluster CCSD(T*)(F12*) harmonic vibrational frequencies and experimental data from the HFREQ2014 dataset, of small first- and second-row polyatomics. G4-T is three times more accurate than plain CCSD(T)/def2-TZVP, while G4-T is two times superior to CCSD(T)/ano-pVTZ. Combining CCSD(T)/ano-pVTZ with MP2-F12 in a parameter-free composite scheme results to a root-mean-square deviation of 5 cm relative to experiment, comparable to CCSD(T) at the complete basis set limit. Application to the harmonic frequencies of benzene reveals a significant advantage of composites with ANO basis sets – MP2/ano-pVmZ and [CCSD(T)-MP2]/ano-pVTZ (m = Q or 5) – over similar protocols based on CCSD(T)/def2-TZVP. Overall, G4-type composite energy schemes, particularly when combined with ANO basis sets in CCSD(T), are accurate and comparatively inexpensive tools for computational vibrational spectroscopy.