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Exciton formation leads to J-bands in solid pentacene. Describing theseexciton bands represents a challenge for both time-dependent (TD)density-functional theory (DFT) and for its semiempirical analogue, namely forTD density-functional tight binding (DFTB) for three reasons (i) solidpentacene and pentacene aggregates are bound only by van der Waals forces whichare notoriously difficult to describe with DFT and DFTB, (ii) the properdescription of the long-range coupling between molecules, needed to describeDavydov splitting, is not easy to include in TD-DFT with traditionalfunctionals and in TD-DFTB, and (iii) mixing may occur between local and chargetransfer excitons, which may, in turn, require special functionals. We assesshow far TD-DFT and TD-DFTB have progressed towards a correct description ofthis type of exciton by including both a dispersion correction for the groundstate and a range-separated hybrid functional for the excited state. Analyticresults for parallel-stacked ethylene are derived which go beyond Kasha'sexciton model in that we are able to make a clear distinction between chargetransfer and energy transfer excitons. This is further confirmed when it isshown that range-separated hybrids have a markedly greater effect oncharge-transfer excitons than on energy-transfer excitons in the case ofparallel-stacked pentacenes. TD-DFT calculations with the CAM-B3LYP functionaland TD-lc-DFT calculations lead to negligeable excitonic corrections for theherringbone crystal structure, possibly because of an overcorrection ofcharge-transfer effects. In this case, TD-DFT calculations with the B3LYPfunctional or TD-DFTB calculations parameterized to B3LYP give the best resultsfor excitonic corrections for the herringbone crystal structure as judged fromcomparison with experimental spectra and with Bethe-Salpeter equationcalculations from the literature.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

Davydov-Type Excitonic Effects on the Absorption Spectra of  Parallel-Stacked and Herringbone Aggregates of Pentacene: Time-Dependent  Density-Functional Theory and Time-Dependent Density-Functional Tight Binding