On the application of phase relationships to complex structures. XXIII. Automatic determination of crystal structures having pseudo-translational symmetry by a modified MULTAN procedure

The MULTAN-80 program has been modified to handle structures having pseudo-translational symmetry. In this case the intensities of reflexions belonging to some index groups are systematically much weaker than others. The program can classify reflexions in groups in the usual way or according to certain index relationships specified by the user. The temperature factor and the scaling factor can then be calculated for each index group separately. The program recognizes the index groups with systematically weak reflexions; phase triplets involving three such reflexions will be eliminated in the SIGMA2 list. Each starting phase set is developed in two stages. During the first stage only the phases of the 'strong' reflexions are generated while in the second stage the phases of the 'weak' reflexions will be derived from phase relationships of the type 'weak-strong-weak'. Finally, an E map using E values before rescaling, rather than the rescaled ones, is calculated. Tests on typical known structures show that the procedure is a very effective one.