Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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   Exciton formation leads to J-bands in solid pentacene. Describing these
exciton bands represents a challenge for both time-dependent (TD)
density-functional theory (DFT) and for its semiempirical analogue, namely for
TD density-functional tight binding (DFTB) for three reasons (i) solid
pentacene and pentacene aggregates are bound only by van der Waals forces which
are notoriously difficult to describe with DFT and DFTB, (ii) the proper
description of the long-range coupling between molecules, needed to describe
Davydov splitting, is not easy to include in TD-DFT with traditional
functionals and in TD-DFTB, and (iii) mixing may occur between local and charge
transfer excitons, which may, in turn, require special functionals. We assess
how far TD-DFT and TD-DFTB have progressed towards a correct description of
this type of exciton by including both a dispersion correction for the ground
state and a range-separated hybrid functional for the excited state. Analytic
results for parallel-stacked ethylene are derived which go beyond Kasha's
exciton model in that we are able to make a clear distinction between charge
transfer and energy transfer excitons. This is further confirmed when it is
shown that range-separated hybrids have a markedly greater effect on
charge-transfer excitons than on energy-transfer excitons in the case of
parallel-stacked pentacenes. TD-DFT calculations with the CAM-B3LYP functional
and TD-lc-DFT calculations lead to negligeable excitonic corrections for the
herringbone crystal structure, possibly because of an overcorrection of
charge-transfer effects. In this case, TD-DFT calculations with the B3LYP
functional or TD-DFTB calculations parameterized to B3LYP give the best results
for excitonic corrections for the herringbone crystal structure as judged from
comparison with experimental spectra and with Bethe-Salpeter equation
calculations from the literature.

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The categories of this paper are: cond-mat
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The categories of this paper are: nlin
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The categories of this paper are: q-alg
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The categories of this paper are: hep-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: stat
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The categories of this paper are: nucl-th
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The categories of this paper are: math-ph
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The categories of this paper are: adap-org
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The categories of this paper are: q-bio
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The categories of this paper are: supr-con
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The categories of this paper are: econ
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The categories of this paper are: cmp-lg
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The categories of this paper are: ao-sci
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The categories of this paper are: plasm-ph
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The categories of this paper are: eess