Accurate Anharmonic Vibrational Frequencies for Uracil: The Performance of Composite Schemes and Hybrid CC/DFT Model.

The vibrational spectrum (frequencies as well as intensities) of uracil has been investigated at a high level of theory. The harmonic force field has been evaluated at the coupled-cluster (CC) level in conjunction with a triple-ζ basis set. Extrapolation to the basis set limit as well as inclusion of core-correlation and diffuse-function corrections have been considered by means of the second-order Møller-Plesset perturbation theory. To go beyond the harmonic approximation, a hybrid CC/DFT approach has been employed, which will be proved to provide state-of-the-art results. As the spectroscopic investigation of uracil is hampered by numerous Fermi resonances, models for explicitly taking them into account have been implemented and applied. On general grounds, the computational procedure presented is able to provide the proper accuracy to support experimental investigations of large molecules of biological interest.