Full-profile Simulated Annealing Method for Solving Complex Molecular Structures from Powder Diffraction Data

School of Chemistry, University of St. Andrews, St. Andrews, Fife KY16 9ST, Scotland

A method of ab initio solution of crystal structures comprising several flexible molecular moieties in the asymmetric unit from powder diffraction is presented. The method utilises a global optimisation technique of simulated annealing to find the minimum of a figure-of-merit function, corresponding to the true structure, based on the quality of the full-profile fit to the experimental pattern. Stereochemical parameters of molecular fragments, bond lengths, bond angles and torsion angles, rather than crystallographic coordinates of individual atoms serve as variables in the minimisation procedure A detailed account of suitable simulated annealing protocols  is given together with our versatile method of describing molecules using the stereochemical descriptors. The technique is illustrated by examples of successful ab initio structure determinations of difficult-to-solve structures.