The structures, vibrational frequencies, electronic properties, and cis-trans photoisomerization process of retinal chromophore and its derivatives (i.e., Schiff base and protonated Schiff base) are studied using the density-functional theory with Becke's three-parameter exchange functional together with the correlation functionals of Lee-Yang-Parr (B3LYP) and the second order Moller-Plesset perturbation theory (MP2). The optical transition energies for photoisomerization are reported at the configuration interaction (CI) level with single excitations (CIS) as well as at the B3LYP/6-31G* level using the random-phase-approximation (RPA). For slightly simplified model systems of retinal chromophore and its derivatives, the RPA values are very close to those of the complete active-space self-consistent-field (CASSCF) method and the multireference CI (MRCI) method, and are also in reasonable agreement with the experiments. We have also tried to investigate the solvent effect of the vertical transition energies in the presence of one or two water molecules. The present study deals with the mechanism of the cis-trans (or trans-cis) photoisomerization based on the molecular orbital (MO) analysis.