PowderSolve– a complete package for crystal structure solution from powder diffraction patterns

Abstract

Powder diffraction techniques are becoming increasingly popular as tools for the determination of crystal structures. The authors of this paper have developed a software package, named PowderSolve, to solve crystal structures from experimental powder diffraction patterns and have applied this package to solve the crystal structures of organic compounds with up to 18 variable degrees of freedom (defined in terms of the positions, orientations, and internal torsions of the molecular fragments in the asymmetric unit). The package employs a combination of simulated annealing and rigid-body Rietveld refinement techniques to maximize the agreement between calculated and experimental powder diffraction patterns. The agreement is measured by a full-profile comparison (using the R factor Rwp). As an additional check at the end of the structure solution process, accurate force-field energies may be used to confirm the stability of the proposed structure solutions. To generate the calculated powder diffraction pattern, lattice parameters, peak shape parameters and background parameters must be determined accurately before proceeding with the structure solution calculations. For this purpose, a novel variant of the Pawley algorithm is proposed, which avoids the instabilities of the original Pawley method. The successful application and performance of PowderSolve for crystal structure solution of 14 organic compounds of differing complexity are discussed.