METHOD OF CRYSTAL-STRUCTURE SIMILARITY SEARCHING

An algorithm is presented which is used to recognize the similarity of crystal structures by matching a description of one structure against the equivalent descriptions of a second. Equivalent descriptions are considered since there are multiple possible ways of choosing the crystal axes and their origin, the asymmetric unit and atom numbering. Another idea used in the algorithm is to use positional and rotational parameters of the molecular fragments to describe the principal crystal structure. The two structures are considered to be similar if in such descriptions the corresponding cell parameters and the parameters of corresponding fragments differ from each other within the limits specified. As a result of such similarity searching, the type of structural similarity, including the transformation matrix of crystallographic axes and the atom correspondences, is determined. The algorithm has been put to practical use in the CRYCOM program. This program is compatible with the Cambridge Structural Database (CSD) through interaction with the GEOM78 program of CSD. The latter program is also used in CRYCOM for recognizing molecular fragments. use of CRYCOM is illustrated in a few examples.