THE APPLICATION OF A GENETIC ALGORITHM FOR SOLVING CRYSTAL-STRUCTURESFROM POWDER DIFFRACTION DATA

We report the successful application of a genetic algorithm to tackle crystal structure solution from powder diffraction data in the case of a previously unknown structure - ortho-thymotic acid. In the structur e solution calculation, the structural fragment was subjected to combi ned translation and rotation within the unit cell, together with varia tion of selected intramolecular degrees of freedom under the control o f a genetic algorithm, in which a population of trial structures is al lowed to evolve subject to well-defined procedures for mating, mutatio n and natural selection. Importantly, the genetic algorithm approach a dopts the 'direct-space' philosophy for structure solution, and implic itly avoids the problematic step of extracting the intensities of indi vidual reflections from the powder diffraction data. The structure sol ution was found efficiently in the genetic algorithm calculation, and was then used as the initial structural model in Rietveld refinement c alculations. The work reported in this Letter paves the way for the fu ture application of the genetic algorithm approach to a much wider arr ay of structural problems. (C) 1997 Elsevier Science B.V.