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Crystal and molecular structures of small-molecule solids solved with the free-distribution software PSSP and WinPSSP

Since the early 2000’s several free-distribution and commercial computer programs have advanced crystal structure determination from powders (SDPD). The free-distribution software WinPSSP [1] consists of a graphical user interface for an essentially unmodified version of PSSP [2], which uses a reconstructed X-ray powder diffraction pattern (a pseudo-pattern ) made of correlated peak intensities calculated from the results of a Le Bail fit [3] (FWHM, peak positions, multiplicity factors and squared structure-factor amplitudes). The simulated annealing algorithm [4] is used to locate the asymmetric unit (provided in Cartesian coordinates) in the unit cell, using a space group symmetry candidate, a cost function (quantitatively evaluating the agreement between the diffracted intensity in the pseudo-pattern and that calculated from a large number of trial models), once a set of structural parameters (fragment positions, Eulerian angles and torsional angles) have been defined by the user. The crystal and molecular structures of more than fifty small-molecule organics have been solved with PSSP and WinPSSP. After Rietveld refinement (typically done with GSAS [5]), those have been published in peer-reviewed journals, or the atomic coordinates have been submitted together with reference powder diffraction patterns to the Powder Diffraction File database [6]. In most cases, synchrotron X-ray powder diffraction from capillary transmission geometry has been used. This work analyses the distribution of various indicators among the structures solved, such as internal, external and total degrees of freedom, space group symmetries, Z, Z’, (2019). Powder Diffraction 34 , 352-360.