The computer code FASCOD2 calculates the atmospheric optical depth with high spectral resolution for different meteorological situations. Since the optical depth and the refractive index are related to respectively the imaginary and real part of the susceptibility, which are interconnected by the Kramers-Kronig relations, it is possible to calculate the atmospheric refractive index by a modified FASCOD2. Due to the computational burden and the lack of well-defined distant wings of the molecular line shape, it is only possible to compute the anomalous dispersion in the neighborhood of the molecular resonance frequencies. This presentation describes briefly a theoretical development of the anomalous dispersion from the optical depth which is valid in a spectral interval ranging from radio waves up to the UV. Line by line calculations of the atmospheric anomalous dispersion at high spectral resolution will be demonstrated for different meteorological situations.