  We have applied a many-body Wannier functions method to theoretically
calculate an excitonic optical conductivity spectrum and energy structure in a
one-dimensional (1D) Mott insulator at absolute zero temperature with large
system size. Focusing on full charge fluctuations associated with pairs of a
holon and doublon, we employ a charge model, which is interpreted as a good
effective model to investigate photoexcitations of a 1D extended Hubbard model
at half-filling in the spin-charge separation picture. As a result, the
theoretical spectra with appropriate broadenings qualitatively reproduce the
recent experimental data of ET-F$_{2}$TCNQ at 294 K with and without a
modulated electric field. Regarding the excitonic energy structure, we have
found that the excitons, especially for even-parity, are weakly bound by
many-body effects. This is also consistent with the fitting parameters reported
in the recent experiment. Thus, our theoretical method presented in this paper
is practically useful to understand physical roles of charge fluctuations in
many-body excited states of a 1D Mott insulator.
