IMPAS–A simple structure determination procedure based on intermolecular Patterson vectors

Abstract

A computer method for interpreting Patterson functions of crystals belonging to space groups of higher symmetry than P1 is described. A table of all Patterson peaks in the entire cell is initially searched for vectors between symmetry-related atoms. Such 'generalized Harker vectors' are identified not only by known symmetry-fixed components, but also by statistical criteria based on a systematic search for pairs of vectors which relate additional atoms to the pair involved in the generalized Harker vector. Subsequent searches, using the same basic principle, are carried out to find additional atoms. Pairs of vectors, whose components add up to the previously established Harker vector, indicate newly found atom positions after appropriate transformations into crystal space. The correctness of these atom positions can be further tested by applying all symmetry operations of the space group in question and looking for a complete set of related vectors. In contrast to many other Patterson search methods no information about known molecular fragments is requested. Also described are four structures which were solved with the IMPAS procedure following unsuccessful attempts to solve them by direct methods.