A Discrete Algorithm for Crystal Structure Prediction of Organic Molecules

A new algorithm, FlexCryst, is presented for fast crystal structure prediction. The algorithm differs from existing algorithms in that it performs the analysis on the basis of only a single molecule and uses potentials for scoring energy that are derived statistically from a set of data on molecular structures. In a first step, the algorithm creates various potential unit cells. In the second step, a set of candidates for translation vectors for corresponding crystals is generated. In the third step, the algorithm selects triples of candidate vectors to form potential crystal structures. The fourth step ranks the crystal structures with respect to their energy as estimated by a suitable scoring function. In the last step, the crystal structures are clustered according to a newly defined measure of similarity for crystal structures. At the moment, the program can handle only triclinic crystals with one molecule per asymmetric unit. The algorithm was tested on a set of 131 experimentally resolved crystals of space group P1 and 95 crystals of space group P{\overline 1} from the Cambridge Structural Database. For P1, in 129 cases (98%), the observed crystal structure is among the crystal structures generated by the algorithm. The run time of the algorithm is a few s per molecule on a standard workstation. For P{\overline 1}, the experimental structure has been found among the proposed structures in 81 cases (85%). Owing to the more complex unit cell for this space group, the run time increases to about 2 h per molecule.