The refractive indices of benzene in the gas and liquid phase are computed employing a classical electrodynamical treatment of light propagation in continuous media and high level ab initio computations of the relevant molecular properties. We investigate various levels of complexity when deriving expressions relating the microscopic molecular optical properties and the refractive index. Different methods of accounting for the molecular environment within the framework of quantum chemical computations are scrutinized. The fundamental molecular property determining the refractivite index is the polarizability, and dynamic molecular polarizabilities of benzene are adequately computed using a triple-zeta plus double polarization basis; larger basis sets change the computed values by less than 1%. The effects of electronic correlation on the dynamic polarizabilities are discussed. For benzene vapor, computed results for the refractive index agree well with experiment. For liquid benzene, the best agreement with experiment is found using the Lorentz-Lorenz formula with computed gas-phase polarizability. We introduce the isolated molecule classical environment (IMLE) and the iterative self-consistent reaction field approach.