Importance of vibrational anharmonicity for electron-phonon coupling in molecular crystals

Accurate predictions of electronic band gaps are key to the computational design of functional materials with tailored optical and thermoelectric properties. While an accurate description of the underlying electronic structure is indispensable, electron-phonon interactions also play a prominent role in determining the band gap. The harmonic vibrational approximation is used almost universally to describe electron-phonon coupling and provides a satisfactory description in most systems. Here, we demonstrate that this is not the case for molecular crystals due to the presence of strongly anharmonic motions. We demonstrate the breakdown of the harmonic approximation in acene molecular crystals, and show that converged band gaps can be obtained by rigorously accounting for vibrational anharmonicity using path-integral molecular dynamics. Finally, we characterize the most anharmonic vibrational modes and their contributions to the band-gap renormalization.