Difference structure-factor normalization for heavy-atom or anomalous-scattering substructure determinations

Procedures are described for normalizing structure-factor difference magnitudes, \Delta\F parallel to(SIR) = parallel to F-Der\ - \F(Nat)parallel to less than or equal to \F-Heavy\ or \Delta\F parallel to(SAS) = parallel to F+h\ - \F(-h)parallel to less than or equal to 2\F "\, to prepare data for probabilistic direct methods phasing to determine heavy-atom or anomalous-scattering substructures in SIR (single-derivative isomorphous replacement) or SAS (single-wavelength anomalous scattering) cases. Applications of the procedures in several recent determinations of multi-selenium substructures in selenomethionyl proteins via SnB direct-methods phasing are briefly summarized.