The refractive indices (linear response) and the nonlinear susceptibility for third harmonic generation (THG) at lambda =1064 nm of liquid water and benzene are predicted, using ab initio molecular (hyper)polarizabilities and local-field corrections calculated according to the Lorentz model, the generalized reaction-field model of Onsager, and a discrete local-field theory applied to liquid structures obtained by classical molecular simulation methods. The ratio of the calculated THG susceptibilities for the two liquids is compared with experimental results, in order to avoid ambiguities concerning the factors used to calibrate the THG experiments. All models are able to reproduce the experimental refractive indices of both liquids accurately, but are less successful for the THG susceptibility. The discrete local-field corrections for the THG susceptibility are considerably larger than those of the continuum model in the form of the more accurate extended reaction-field approach.