Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and Site-Dependent van der Waals Interaction.

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift Δ[Formula: see text] is mainly caused by dispersion effects and depends sensitively on the molecule's specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide Δ[Formula: see text] = - in two factors, where depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while is a geometry factor expressing the site dependence of the shift in a given molecular structure.