Anharmonic corrections to the fundamental frequencies, as well as first overtones and combination band frequencies were calculated from the cubic and quartic force constants, the rotational constants and the Coriolis coupling constants using a scheme based on vibrational second-order perturbation theory35,36 as described in Section 2.2, where terms found to be affected by Fermi resonances are taken out of the perturbational treatment23 and resolved variationally41 using the GVPT2 model.38