Enhanced Efficiency of Direct-Space Structure Solution from Powder X-ray Diffraction Data in the Case of Conformationally Flexible Molecules

A strategy is reported for assessing the feasibility of molecular conformations within direct-space structure-solution calculations of organic molecular crystal structures from powder X-ray diffraction data, focusing in particular on the genetic algorithm technique for structure solution in which fitness is defined as a function of the whole-profile figure-of-merit R_wp. The strategy employs a readily computed distance-based function to assess the feasibility of the molecular conformation in each trial structure generated in the genetic algorithm calculation, and structures considered to have low-feasibility conformations are penalized within the evolutionary process. The strategy is shown to lead to significant improvements in the success rate of structure-solution calculations in the case of flexible molecules with a significant number of conformational degrees of freedom.