Solution of organic crystal structures from powder diffraction by combining simulated annealing and direct methods

The ab-initio crystal structure solution from powder diffraction data can be attempted via direct methods. If heavy atoms are present, they are usually correctly located: then some crystal chemical information can be exploited to complete the partial structure model. Organic structures are more resistant to direct methods: as an alternative, their molecular geometry is used as prior information for Monte Carlo methods. In this paper a new procedure is described which combines the information contained in the electron density map provided by direct methods with a Monte Carlo method which uses simulated annealing as minimization algorithm. A figure of merit has been designed based on the agreement between the experimental and calculated profiles, and on the positions of the peaks in the electron density map. The procedure is completely automatic and has been included in EXPO; its performance has been validated and tested for a set of known molecular structures.