  Charge ordering (CO) with long-periodicities in a geometrically frustrated
system, theta-(BEDT-TTF)2X, is numerically studied. We consider interacting
fermion models on the anisotropic triangular lattice: the extended Hubbard
model and the spinless fermion model, taking account of the inter-site Coulomb
interactions on not only the nearest neighbor (NN) but also the next nearest
neighbor (NNN) bonds. By applying Lanczos exact diagonalization method, we find
that long-period charge correlations become dominant when NNN terms are
included, in the charge-frustrated region. The calculated optical excitation
spectra show that the charge transfer excitations in the long-period CO extend
to a lower energy range compared to those in the short-period CO states
including the 3-fold CO stabilized when NNN terms are absent. Our results
suggest that the charge correlations experimentally observed in the title
compounds near the CO insulating phase can be viewed as a tendency toward
`tilted stripe'-type CO with large charge fluctuations.
