The code interspectus, described in the article Phys. Rev. B 79, 094102 (2009), performs the interpolation between positive-definite spectra that obey a sum rule. Examples are the spectra of electron energy-loss spectroscopy for varying momentun transfer or the absorption spectra of nanocrystals with varying size. For a description of the interpolation method, please refer to the above-mentioned article. For any work in which the code has been used, please cite this article.
The code comes in two versions. There is
Both come with lots of explanations and examples. Both are free software.