  We propose an effective model called the "charge model", for the half-filled
one-dimensional Hubbard and extended Hubbard models. In this model, spin-charge
separation, which has been justified from an infinite on-site repulsion ($U$)
in the strict sense, is compatible with charge fluctuations. Our analyses based
on the many-body Wannier functions succeeded in determining the optical
conductivity spectra in large systems. The obtained spectra reproduce the
spectra for the original models well even in the intermediate $U$ region of
$U=5-10T$, with $T$ being the nearest-neighbor electron hopping energy. These
results indicate that the spin-charge separation works fairly well in this
intermediate $U$ region against the usual expectation and that the charge model
is an effective model that applies to actual quasi-one-dimensional materials
classified as strongly correlated electron systems.
