Robust and user-friendly models beyond the harmonic level of theory for vibrational spectroscopy

Vibrational spectroscopy represents the tool of choice for the characterization of molecular systems, in particular the univocal determination of their structures. The interpretation of most experimental spectra is difficult due to their inherent complexity and computational spectroscopy has shown to be a valuable tool to help unravel the various contributions to the spectrum, allowing for a better understanding of the underlying phenomena. However, most calculations are still limited to the harmonic oscillator level. In this context, we present a robust and versatile model to compute vibrational properties at the anharmonic level, using second order perturbation theory. In particular, a consistent method is used to overcome the well-known problem of Fermi resonances in vibrational energy calculations. The anharmonic correction is also applied to intensity calculations, allowing us to compute infrared, vibrational circular dichroism and Raman spectra. The overall quality of the approach is shown in comparison with experimental results.