EPCryst: a computer program for solving crystal structures from powder diffraction data

Recently, a new algorithm has been proposed to generate a full set of trial structure models (TSMs) by enumerating all possible equivalent position combinations (EPCs) based on the unit-cell content and space-group information [Deng & Dong (2009). J. Appl. Cryst. 42, 953-958]. Using this algorithm, a new computer program named EPCryst has been developed for crystal structure determination from powder diffraction data. It is designed to solve a crystal structure as efficiently as possible. Rather than applying a time-consuming global optimization procedure on each TSM, EPCryst firstly tries to eliminate improbable TSMs. Several methods (such as the statistical analysis method and direct-space heavy-atom method) are designed to achieve this goal. Usually, a lot of improbable TSMs can be eliminated and only a few promising TSMs are preserved for global optimization by grid search or simulated annealing. These methods can greatly increase the efficiency of structure solution in direct space. Bond-length checking is available in EPCryst as a tool for crystal structure validation. EPCryst was successfully applied to determine several inorganic crystal structures using powder diffraction data obtained from PowBase [Le Bail (2000). http://sdpd.univ-lemans.fr/powbase/ ].