Solution of flexible structures from powder diffraction data using a simulated annealing technique

The available techniques of structure solution from powder diffraction data are not optimised for addressing the problem of flexible molecules which account for the vast majority of molecular crystal structures. In response we have developed a new technique based on simulated annealing. Our approach involves generating trial structures by making random changes in stereochemical descriptors (bond lengths, bond angles and torsion angles), transforming these to atomic coordinates in the crystallographic frame of reference, from which a powder profile and hence a goodness-of-fit are calculated. Using the quality of the full-pattern fit as a criterion, the simulated annealing method locates the global minimum by performing a step-wise scan of the multidimensional space of the stereochemical descriptors in the Monte Carlo fashion. The technique is illustrated by solving structures comprising various types of flexible moieties along with individual atoms.