Fast clustering of equivalent structures in crystal structure prediction

Most methods of crystal structure prediction generate many trial structures. Because these may differ in choice of unit cell, it is not always immediately obvious whether or not two such structures are equivalent. A method to answer this question is described for the case where the asymmetric unit contains one molecule in a general position, defined by the rotation and translation of that molecule with respect to some reference geometry. In the comparison of two structures, the rotation needed to transform one orientation into the other is determined first. Then it is checked whether this rotation corresponds to a transformation that is compatible with the imposed space group symmetry. A final test compares the cell lengths, the cell angles, and the molecular centers of gravity after the transformation of one structure into the other. The method is implemented for triclinic, monoclinic, and orthorhombic systems and is found to be very fast in tests on hypothetical crystal structures of acetic acid. © 1997 John Wiley & Sons, Inc. J Comput Chem 18:1036–1042, 1997

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