Assessment of CCSD(T)-F12 Approximations and Basis Sets for Harmonic Vibrational Frequencies

We consider basis set convergence and the effect of various approximations to CCSD­(T)-F12 for a representative sample of harmonic frequencies (the HFREQ2014 set). CCSD­(T*)­(F12*)/cc-pVDZ-F12 offers a particularly favorable compromise between accuracy and computational cost: its RMSD <3 cm–1 from the valence CCSD­(T) limit is actually less than the remaining discrepancy with the experimental value at the valence CCSD­(T) limit (about 5 cm–1 RMSD). CCSD­(T)-F12a and CCSD­(T)-F12b appear to benefit from error compensation between CCSD and (T).