Accurate calculation of vibrational frequencies using explicitly correlated coupled-cluster theory

The recently proposed explicitly correlated CCSD (T) -F12x (x=a,b) approximations [T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys. 127, 221106 (2007)] are applied to compute equilibrium structures and harmonic as well as anharmonic vibrational frequencies for H2 O, HCN, CO2, CH2 O, H2 O2, C2 H2, CH2 NH, C2 H2 O, and the trans-isomer of 1,2- C2 H2 F2. Using aug-cc-pVTZ basis sets, the CCSD(T)-F12a equilibrium geometries and harmonic vibrational frequencies are in very close agreement with CCSD(T)/aug-cc-pV5Z values. The anharmonic frequencies are evaluated using vibrational self-consistent field and vibrational configuration interaction methods based on automatically generated potential energy surfaces. The mean absolute deviation of the CCSD(T)-F12a/aug-cc-pVTZ anharmonic frequencies from experimental values amounts to only 4.0 cm-1.