Subtleties in Crystal Structure solution from powder diffraction data using Simulated Annealing Method

Recent advances in crystallographic computing and availability of high-resolution diffraction data have made it relatively easy to solve crystal structures from powders that would have traditionally required single crystal samples. The success of direct space methods depends heavily on the accuracy of the starting molecular model. In this paper we address the applicability of using these methods in finding subtleties such as disorder in the molecular conformation of a crystal that might not be known a priori. We use Ranitidine HCl as our test sample as it is known to have a conformational disorder from single crystal structural work. We re-determine the structure from powder data using simulated annealing method and argue that the unusually large thermal parameters obtained from a Rietveld type analysis points us to the error in the incorrect starting model.