FAST CLUSTERING OF EQUIVALENT STRUCTURES IN CRYSTAL-STRUCTURE PREDICTION

Most methods of crystal structure prediction generate many trial struc tures. Because these may differ in choice of unit cell, it is not alwa ys immediately obvious whether or not two such structures are equivale nt. A method to answer this question is described for the case where t he asymmetric unit contains one molecule in a general position, define d by the rotation and translation of that molecule with respect to som e reference geometry. In the comparison of two structures, the rotatio n needed to transform one orientation into the other is determined fir st. Then it is checked whether this rotation corresponds to a transfor mation that is compatible with the imposed space group symmetry. A fin al 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 orthor hombic systems and is found to be very fast in tests on hypothetical c rystal structures of acetic acid. (C) 1997 by John Wiley & Sons, Inc.