Spectral dependence of the real ε1 (ω) and imaginary ε2 (ω) parts of the complex dielectric function, complex refractive index, absorption coefficient, and normal-incidence reflectivity of Cu Ga5 Se8 crystals with slightly different Cu contents are modeled using Adachi's model for interband transitions. The results are in good agreement with the experimental data over the entire range of photon energies. The model parameters are determined using the simulated annealing algorithm.