Infrared optical properties of amorphous polymers, polymethylmethacrylate and polychlorotrifluoroethylene, are predicted following a procedure based on molecular simulation and Kramers-Kronig relations. The amorphous phase of polymer is first simulated with periodic boundary conditions. Normal mode vibrations and integrated intensities are then extracted from simulation data, These parameters allow the computation of the imaginary part, k, of the complex refractive index, N. Using Kramers-Kronig relations, the real part, n, of N, can be techniques, spectrometry and ellipsometry. Three observations follow from such a comparison. Firstly, the ellipsometry is the most appropriate technique for this specific measurements. Secondly, some problems still exist in the intensity determination in the low frequency region. Thirdly, the mid-IR region is accurately depicted.