The method chosen in the present work is the so-called generalized vibrational second-order perturbation (GVPT2) model.38,41 In this method, a VPT2 treatment of the molecular vibrations is used, except for the cases where Fermi resonances are considered to have occurred. In these cases, the terms in the VPT2 treatment that are affected by the Fermi resonance are not included,23 and the affected frequencies are instead resolved in a variational approach.