Section 25.2.1.1.1. Isomorphous replacement, anomalous scattering and MAD phasing

Heavy-atom-based phasing by the methods of isomorphous replacement (Green et al., 1954) and/or anomalous scattering (Pepinsky & Okaya, 1956) are initiated by reading one or more `scaled' files into the program PHASIT, along with estimates of the heavy-atom or anomalous-scatterer positional, occupancy and thermal parameters. Each input file can contain either isomorphous-replacement, derivative anomalous-scattering or native anomalous-scattering data. MAD (multiple-wavelength anomalous diffraction) data are treated as both isomorphous and anomalous-scattering data, in which case one simply inputs the scattering-factor differences (real part) appropriate for the wavelengths comprising the `isomorphous' data sets and the actual scattering factors (imaginary part) appropriate for the `anomalous' data sets. All possible combinations of isomorphous-replacement data, conventional anomalous-scattering data and MAD data are allowed and can be used simultaneously during phasing.