Calculating optical absorption spectra of thin polycrystalline 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 $\Delta{\cal E}_m$ 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 $\Delta{\cal E}_m= -Q W_{m}$ in two factors where $Q$ depends only on the molecular species and accounts for all non-resonant electronic transitions contributing to the dispersion, while $W_m$ is a sum running over the position of all molecules expressing the site-dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylene-tetracarboxylic-diimide (PTCDI). 
J\"org Megow, Thomas K\"orzd\"orfer, Thomas Renger, Mino Sparenberg,   Sylke Blumstengel, Fritz Henneberger, and Volkhard May
cond-mat.mtrl-sci cond-mat.mes-hall physics.chem-ph