Vibrational spectroscopy is a powerful investigative tool for the study and determination of the structural and dynamical properties of molecular systems. With the advent of experimental techniques able to provide well-resolved band-shapes, more detailed information on those properties becomes accessible. This gain in accuracy necessitates adapted tools to ensure the correct identification and a reliable assignment of the observed bands. As a result, computational spectroscopy is now routinely used to support the analysis of experimental data and to guide the discovery of new findings. However, the reliability and accuracy of theoretical calculations, in particular for sensitive spectroscopies like vibrational circular dichroism (VCD) and Raman optical activity (ROA), depends strongly on the accuracy of the underlying theory. Thanks to progresses in algorithms and hardware performance, models beyond the harmonic level of approximation are now applicable to medium-large molecular systems of several dozens of atoms. In particular, second-order vibrational perturbation theory (VPT2) is an appealing method, since it offers a significant improvement in accuracy and reliability over harmonic, including scaled, results for a reasonable raise of the computational cost.<br/>In this contribution, we will present the general aspects of VPT2 applied to the calculation of transition energies and intensities of chiroptical spectra, and show how fully anharmonic spectra can lead to a better understanding of their structure. The reliability of the results not only derives from a proper choice of the electronic structure calculation method but also from a careful assessment of limitations and shortcomings of the theory employed for the vibrational part. With the aim of providing robust methods to build automatic procedures and facilitate such calculations, we will also address the well-known problem of resonances in VPT2, their impact on band positions and intensities, and the strategies available to overcome them.<br/>Finally, data produced during simulations can be used to get better insights on the origin of each band. By choosing a suitable graphical representation, they can provide a visual aid for a detailed analysis of the spectra. Moreover, they can help set up reliable reduced-dimensionality schemes to be applied to larger systems where a full anharmonic treatment would be too expensive.

Mechanical, electrical and magnetic anharmonicity in chiroptical vibrational spectra: The VPT2 approach

255th National Meeting (2018)

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