By leaving the double harmonic approximation, it is possible to obtain a more accurate description of both the vibrational frequencies and the spectroscopic intensities.23,35–38 For the latter, the introduction of anharmonic effects will enable both an improved description of the intensities associated with a single excitation of a particular normal mode as well as introduce the leading-order contributions to intensities associated with transitions corresponding to the simultaneous excitation of two or more vibrational quanta, either involving only a single normal mode or several of them, often referred to as overtone and combination bands, respectively.