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 our current implementation of this approach, the structure solution process involves exploring a powder-profile R-factor hypersurface to find the global minimum with respect to the set (Γ) of structural variables that define the 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] search algorithm, implemented in the program EAGER [3]. Conventionally, the structural variables in the set Γ comprise, for each molecule in the asymmetric unit, the position and orientation of the molecule with respect to the unit cell axes, and a set of variable torsion angles. Here we introduce an alternative approach, in which the standard set of structural variables is transformed to an alternative set of variables of the same or lower dimensionality, with this transformation carried out in a dynamic manner within the evolution of the GA calculation. Here we assess the feasibility of this approach in comparison with the standard approach. Structures containing highly flexible molecules defined by a significant number of torsional degrees of freedom represent a challenging case for direct-space structure solution [4], and our exploration of the potential advantages of altering the variable-space dynamically during GA structure solution calculations is focused in particular on examples of this type of structural problem.

[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] Z. Zhou, S. Habershon, 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] A.J. Hanson, E.Y. Cheung, K.D.M. Harris, J. Phys. Chem. B 2007, 111, 6349.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/16239 must be provided.

1. On the definition of variable-space in direct-space structure solution from powder diffraction data using Genetic Algorithms
2. Optimization and applications of genetic algorithms for structure determination from powder diffraction data