Resolution Bias Correction in EXPO209

Fourier syntheses providing electron density maps always show series-termination errors due to the limited experimental resolution. The worse the resolution, the worse the quality of the map: peaks are misplaced with respect to the true positions, positive and negative peaks occur, the peak profile is deformed. Two new algorithms have been developed aimed at reducing the resolution bias. The first one works in direct space [1,2], the second one in reciprocal space [3]. They are able to improve both peak positions and peak form in such a way that the map better fits the real atomic electron density. The new procedures have been introduced in EXPO2009, the strengthened and updated version of EXPO [4] and applied to the electron density maps provided by Direct Methods. Organic as well as metallorganic powder structures with non-atomic resolution experimental data have been tested. In spite of the difficulties in structure solution by powder data, the new approaches succeeded in several cases which could not be ,solved without the resolution bias correction. The theory of the resolution bias correction and its application by EXPO2009 will be presented.