Density-of-states of crystalline 2, 2'-bithiophene: ab initio analysis and comparison with inelastic neutron scattering response

Phonons in the crystalline phase of 2, 2'-bithiophene (2T) are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. Phonon calculations are performed as a function of the long range interactions and the displacement amplitude used for calculating Hellmann-Feynman forces. For the first time, we show that both these parameters are crucial in simulating accurately the experimental low-frequency density-of-states of the 2T crystalline phase, obtained from inelastic neutron scattering experiments. The DFT/direct method approach allows the anharmonicity of phonons to be investigated. The anharmonic behaviour of the low-frequency vibrational modes (below 300 cm-1) as the high-frequency vibrational modes at 685, 800 and 885 cm-1 of the 2T crystalline phase is clearly demonstrated. Except for these three latter high-frequency modes, a harmonic behaviour is observed for the intramolecular modes. The calculation of the multiphonon contribution predicts the appearance of a broad background in the 600-1500 cm-1 frequency range, as well as defined features around 950 and 1130 cm-1, in good agreement with the inelastic neutron scattering data. Finally, low-frequency dispersion curves are given