Dispersion of vibrational modes in benzoic acid crystals

The k-vector dispersion of vibrational excitations in benzoic acid crystals was calculated, based on force constants determined from periodic DFT calculations in a simulation box composed of four unit cells of the crystal. To compensate for the underevaluation of intermolecular attractive forces in DFT calculations, the geometry of the unit cell, measured at low temperature, was imposed. Inelastic neutron spectra, determined from the corresponding density of states, agree well with measured spectra in the region of the in-plane and out-of-plane bending modes of the carbonyl group, where the dispersion is noticeable. This result makes unnecessary alternative explanations, based on unphysical assumptions, for the structure observed in the spectra. The good agreement between experiment and calculation validates the approach used here to determine the intermolecular potential.