Peak labelling in electron density maps from powder data: the use of crystal chemical information

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 as POLPO [Altomare et al . (2000). J. Appl. Cryst. 33 , 1305–1310], requires that all the heavy atoms be positioned and exactly labelled.