Despite progresses made in the field of computational spectroscopy and the improvements of hardware resources, vibrational spectra are still commonly done at the harmonic level. Further refinement is then limited to the introduction of a scaling factor to the harmonic frequencies. However, the reliability of this correction is likely to be inconsistent over the spectrum window. While the use of multiple factors can help improving the results, it also introduces a strong empirical bias to ab initio simulations.<br/>In order to obtain accurate results, a proper account of the anharmonic effects is necessary. In this context, second-order vibrational perturbation theory (VPT2) is particularly appealing for the accuracy it provides at a reasonable computational cost. While extensive work has been devoted to the proper calculation of vibrational energies at the VPT2 level, this has not been the case for intensities, which are often overlooked. This situation has changed in the recent years with the development of new, simpler equations and the availability of programs to compute spectra at a full anharmonic level, including transition intensities, which allows a direct comparison with the experimental ones.<br/>In this contribution, we will illustrate our work in this domain, and in particular the development and application of a versatile and efficient tool able to simulate a large variety of vibrational spectra (ex. IR, VCD, Raman, ROA). This has been made possible by adopting general formulas, greatly facilitating the support of new properties. Emphasis has also been put on the ease of use, so that reliable simulations can be achieved with a minimum input, or very finely tuned through a large range of available options and models. In particular, multi-scale approaches give the possibility to tailor the accuracy and efficiency based on time and hardware resources, either by using very accurate harmonic data or limiting the anharmonic treatment to a specific region of the system.<br/>The general framework and some of its key features will be discussed and illustrated with two prototypical systems: naphthalene, the smallest member of the family of polycyclic aromatic hydrocarbons, and methyloxirane, a common model for chiral molecules. The availability of accurate experimental data for those systems is particularly appealing in order to evaluate the reliability of VPT2 calculations, including the simulation of overtones and combination bands.

Vibrational spectra of medium-to-large systems at the anharmonic level

251st National Meeting (2016)

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