Many solids can be prepared only as microcrystalline powders and are therefore not suitable for structural characterization by conventional single-crystal X-ray diffraction techniques. For such materials, it is necessary to tackle structure determination using powder X-ray diffraction data [1]. Although traditional techniques for structure solution from powder X-ray diffraction data have been applied successfully in many cases, organic molecular materials represent a particularly challenging case. For these reasons, we are focusing on the development, implementation and optimization of new strategies for structure solution from powder X-ray diffraction data, with particular emphasis on molecular crystal structures. Our current work in this field is based on the direct-space strategy [2], in which a hypersurface defined by the powder profile R-factor is searched using a Genetic Algorithm [3,4]. In addition to applying this technique to tackle structure determination for a range of materials of academic and industrial interest, we are carrying out systematic explorations to find the optimum ways of implementing the Genetic Algorithm technique within this field [5,6]. The lecture will review fundamentals of the implementation of Genetic Algorithms in structure solution from powder diffraction data, and recent results from the application of this technique to determine crystal structures of molecular materials will be presented.

[1] K.D.M. Harris, E.Y. Cheung, Chem. Soc. Rev., 2004, 33, 526.

[2] K.D.M. Harris, M. Tremayne, P. Lightfoot, P.G. Bruce, J. Am. Chem. Soc., 1994, 116, 3543.

[3] B.M. Kariuki, H. Serrano-González, R.L. Johnston, K.D.M. Harris, Chem. Phys. Lett., 1997, 280, 189.

[4] K.D.M. Harris, R.L. Johnston, B.M. Kariuki, Acta Crystallogr., 1998, A54, 632.

[5] K.D.M. Harris, S. Habershon, E.Y. Cheung, R.L. Johnston, Z. Kristallogr., 2004, 219, 838.

[6] Z. Zhou, V. Siegler, E.Y. Cheung, S. Habershon, K.D.M. Harris, R.L. Johnston, ChemPhysChem, 2007, 8, 650.

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1. Dynamic re-definition of variable space during direct-space structure solution from powder X-ray diffraction data
2. On the definition of variable-space in direct-space structure solution from powder diffraction data using Genetic Algorithms