To describe the properties of organic semiconductors (OSCs), three different approaches are available, each with its own advantages and disadvantages. Small cluster calculations have achieved great success in modeling photo-induced relaxation processes in OSCs , but they fail miserably in describing absorption phenomena. Periodic boundary conditions typically describe absorption spectra very well , but they cannot simulate photo-induced processes. Finally, theoretical methods based on effective Hamiltonians provide detailed assignments of spectra , but they cannot predict new materials as many of the necessary parameters rely on available experimental data. We have developed a computational protocol based on the cluster approach that provides highly accurate (polarization-resolved) absorption spectra of thin films and crystals of organic semiconductors, such as Pentacene , Tetracene (Figure 1), and a squaraine compound. The key components of our protocol include the use of optimally tuned functionals  and the selection of clusters that reflect the essential symmetries of the crystal structure and allow for delocalized excitons. Comparisons between calculated and measured spectra (e.g., Figure 1) demonstrate that our protocol accurately accounts for both electronic and vibrational effects.
Figure 1: Comparison of the calculated and measured (a) full and (b) polarization-resolved absorption spectrum of Tetracene.
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