Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A Simple Hubbard Model for the Excited States of $\pi$ Conjugated -acene
  Molecules   In this paper we present a model for the electronic excited states of $\pi$
conjugated -acene molecules such as tetracene, pentacene, and hexacene. We use
a simple Hubbard model with a limited basis to describe the low lying
excitations with reasonable quantitative accuracy. We are able to produce
semi-analytic wavefunctions for the electronic states of the system, which
allows us to compute the density correlation functions for various states such
as the ground state, the first two singly excited states, and the lowest lying
doubly excited state. We show that in this lowest lying doubly excited state, a
state speculated to play a role in the singlet fission process, the electrons
and holes behave in a triplet-like manner. We also compute the two-photon
absorption of these -acenes, and show that per number density it is comparable
to that of other organic molecules such as coronene and
hexa-peri-hexabenzocoronene (HBC).

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The categories of this paper are: math-ph
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The categories of this paper are: stat
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The categories of this paper are: alg-geom
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The categories of this paper are: astro-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: q-fin
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The categories of this paper are: hep-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-th
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The categories of this paper are: adap-org
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The categories of this paper are: comp-gas
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The categories of this paper are: atom-ph
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The categories of this paper are: eess
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The categories of this paper are: math
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The categories of this paper are: chao-dyn
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The categories of this paper are: funct-an