Real space technique applied to crystal structure determination from powder data

Real-space techniques used for phase extension and refinement in the modern direct procedures for ab initio crystal structure solution of proteins have been optimized for application to powder diffraction data. The new process has been implemented in a modified version of EXPO [Altomare et al. (1999). J. Appl. Cryst. 32, 339-340]. The method is able to supply a structural model that is more complete than that provided by the standard EXPO program. The model is then refined via a diagonal least-squares procedure, but only when the ratio of the number of observations to the number of structure parameters to be refined is larger than a given threshold.