Ab initio periodic modelling of the vibrational spectra of molecular crystals: the case of uracil

The structure and vibrational spectra of solid uracil have been simulated in the framework of Density Functional Theory (DFT) using a periodic unit cell model. Structural parameters are reproduced reasonably well by using the dispersion corrected, global hybrid Hartree-Fock/DFT functional B3LYP-D* and an all-electron, Gaussian type, triple zeta basis set with polarisation. The periodic calculation provides the full set of fundamental harmonic vibrational modes, whose nature can be investigated by inspecting the corresponding eigenvectors. Accounting for dispersive interactions indirectly affects the spectra, through the impact on the cell parameters. Marked differences are found between the gas and solid phase spectra, that can be related to either mode coupling or direct alteration of the potential energy via neighbour-neighbour molecular interactions. Anharmonicity needs to be considered for a meaningful comparison with experiments; a single scaling factor provides a significantly improved agreement for most of the frequencies, except for the NH stretchings, which require a larger downscaling. This rescaling strategy yields results of comparable quality with respect to previously reported calculations with a cluster model and a perturbative treatment of anharmonicity.