• computer programs;
  • fluorescence spectra;
  • anomalous scattering.

A Fortran program CHOOCH, which derives experimental values of the anomalous-scattering factors f′′ and f′ from X-ray fluorescence spectra, is described. The program assumes knowledge of theoretical values for the imaginary term, f′′, of the anomalous-scattering factor away from the absorption edge to scale the experimental fluorescence spectrum and thus derive values of f′′ near the absorption edge, where tabular data are inappropriate. The Kramers–Kronig relation is used to calculate the real part, f′, of the anomalous-scattering factor. The program aids the decision-making process in macromolecular crystallographic experiments where optimal wavelength selection is required. Magnitudes of f′ and f′′ at selected wavelengths can later be used as starting values for heavy-atom refinement with crystallographic phasing programs.