Structure determination of organic molecular solids from powder X-ray diffraction data is nowadays carried out widely, in particular using the "direct-space strategy" for structure solution [1]. In this approach, the structure solution process involves exploring a powder-profile R-factor hypersurface R_wp(Γ) to find the global minimum, where Γ represents the set of variables that define trial structures. In principle, any technique for global optimization may be used, and our own current work in this field is focused on the use of a Genetic Algorithm (GA) [2], implemented in the program EAGER [3]. Conventionally, the structural variables in the set Γ comprise, for each molecule in the asymmetric unit, the position {x, y, z} and orientation {θ, φ, ψ} of the whole molecule, and a set of n variable torsion angles {τ₁, τ₂, ..., τ_n}. Hitherto, all implementations of direct-space techniques for powder structure solution have defined molecular structures in terms of the variables in the set Γgiven above, with each molecule in the asymmetric unit defined as a complete molecule. This approach is challenged here by an alternative definition of variable-space in which the molecule is broken into two or more smaller fragments by breakage of bonds, and which is demonstrated to be strikingly successful in solving the structures of conformationally complex molecules [4]. Here we assess the question of how the definition of variable space controls the evolutionary progress of GA calculations in this field.

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

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

[3] S. Habershon, Z. Zhou, G.W. Turner, B.M. Kariuki, E.Y. Cheung, A.J. Hanson, E. Tedesco, R.L. Johnston, K.D.M. Harris, EAGER, Cardiff University and University of Birmingham

[4] Z. Zhou, V. Siegler, K.D.M. Harris, E.Y. Cheung, S. Habershon, 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. Optimization and applications of genetic algorithms for structure determination from powder diffraction data