Multiple-fragment representations of molecular geometry in direct-space structure solution from powder X-ray diffraction data using genetic algorithms

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. In our implementation of this approach, structure solution involves the use of a genetic algorithm to explore the powder-profile R-factor hypersurface Rwp(Γ) to locate the global minimum, where Γ represents the set of variables that define trial structures. Conventionally, the set of variables comprises, for each molecule in the asymmetric unit, the position {x, y, z} and orientation {θ, φ, ψ} of the molecule, and a set of n variable torsion angles {τ1, τ2,...,τn}. An alternative definition of variable-space has been explored recently, based on a multiple-fragment representation of molecular geometry, and has been demonstrated to be successful in solving the structures of conformationally complex molecules. This paper explores details of the methodology for the definition of multiple-fragment representations of molecular geometry within the context of the application of the direct-space genetic algorithm technique for structure solution.