Benchmarking calculations of spectral densities for the diagonal and nondiagonal exciton-phonon coupling of tetracene crystal

The spectral densities of diagonal and nondiagonal exciton-phonon (e-p) coupling for tetracene crystal have been calculated by the harmonic oscillator (HO) model and ground-state MD-based approaches. We find that classical MD-based approaches overestimate the coupling of exciton with high-frequency vibrational modes and predict the strongest e-p coupling appeared above 1500 cm⁻¹ whereas HO model and AIMD-based approach predict it appeared at ∼1400 cm⁻¹. Additionally, the calculated spectral densities of nondiagonal e-p coupling for three different dimers show that they are continuously distributed in the range of 0–150 cm⁻¹ and are 2–3 order of magnitude smaller than the maxima of diagonal e-p coupling.