Direct methods applied to powder diffraction data often provide electron density maps of which the quality is usually affected by systematic and/or random phase errors, by amplitude truncation effects in the series representation of the electron density,etc. The frequent incorrect labelling of the peaks can strongly affect the efficiency of the procedures used for crystal structure refinement. For example, the success of alternative techniques, such asPOLPO[Altomareet al. (2000).J. Appl. Cryst.33, 1305–1310], requires that all the heavy atoms be positioned and exactly labelled.