Peierls and Holstein carrier-phonon coupling in crystalline rubrene

A computational protocol for the calculation of local (Holstein) and nonlocal (Peierls) carrier-phonon coupling in molecular organic semiconductors is presented and applied to orthorhombic rubrene (5,6,11,12-tetraphenyltetracene). In the phonon description, the rigid molecule approximation is removed, allowing mixing of low-frequency intramolecular modes with intermolecular (lattice) phonons. Notwithstanding, a rather clear distinction remains between the low-frequency phonons, which essentially modulate the transfer integral from a molecule to another (Peierls coupling), and the high-frequency, fully intramolecular phonons, which modulate the on-site energy (Holstein coupling). The implications for the current models of mobility are shortly discussed.