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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.

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