Ab initio crystal structure solution by powder diffraction data is based on the experimental full pattern decomposition process: the resulting structure factor moduli are used for direct phasing. The extracted intensity estimates are scarcely accurate (overlapping, background, and preferred orientation are the main causes of the lack of accuracy), no matter if Le Bail or Pawley method is adopted: consequently the structure solution process is not straightforward. We have focused our attention on the relation between the efficiency of the EXPO2004 phasing process and the various parameters which are normally used in the full pattern decomposition process, e.g., the peak shape function, the number of refinement cycles, the degree of overlapping. Different steps of the phasing process are considered: the definition of the unit cell, the determination of the space group, and the application of direct methods.
