The influence of intrinsic defects on optical properties of lithium niobate studied using an electronic-structure simulation technique. The simulation technique is based on molecular dynamics cluster geometry optimization, nonlocal pseudopotential calculation, and Green function electron-phonon interaction. Two models of defects are considered. The first one (electrostatically highly imbalanced) corresponds to mixture of oxygen vacancies and Nb-Li antisites. The second one corresponds to the Nb antisites charge compensated by appropriate vacancies, lithium vacancies, and corresponds to more realistic defect configuration. We present a band energy approach with a molecular dynamics cluster optimization that accounts for the various structural modifications related to the nonstoichiometry of LiNbO3 crystals. The variation of the optical properties with the deviation from the stoichiometric composition can be understood within this approach. Especially, the role of the electron-phonon contributions to the electrooptics coefficient is shown. Comparison of the first and second models shows that the virtual model corresponding to mixture of oxygen vacancies and Nb-Li antisites is more suitable for describing of the optical properties. In particular, model calculations yield a large dependence of the electrooptics coefficient r(22) on the crystal composition, in agreement with the experimental data. The observed minimum of the r(22) coefficient versus the nonstoichiometry in the plot is interpreted as being by a diminishing in the noncentrosymmetry of the electrostatic potential around Nb-O-6 clusters.