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

Abstract

Phonons in the crystalline phase of 2,-bithiophene (2T) are investigated using the direct method combined with densityfunctional theory (DFT)-based total energy calculations. Phonon calculations are performedas a function of the long range interactions and the displacement amplitude used forcalculating Hellmann–Feynman forces. For the first time, we show that both theseparameters are crucial in simulating accurately the experimental low-frequencydensity-of-states of the 2T crystalline phase, obtained from inelastic neutron scatteringexperiments. The DFT/direct method approach allows the anharmonicity of phonons to beinvestigated. The anharmonic behaviour of the low-frequency vibrational modes (below300 cm−1) as the high-frequency vibrational modes at 685, 800 and885 cm−1 of the 2T crystalline phase is clearly demonstrated. Except for these three latter high-frequencymodes, a harmonic behaviour is observed for the intramolecular modes. The calculation ofthe multiphonon contribution predicts the appearance of a broad background in the600–1500 cm−1 frequency range, as well as defined features around 950 and1130 cm−1, in good agreement with the inelastic neutron scattering data. Finally, low-frequencydispersion curves are given.