PROBABILISTIC ESTIMATION OF THE STRUCTURE SEMINVARIANTS IN THE MONOCLINIC AND ORTHORHOMBIC SYSTEMS WHEN ANOMALOUS SCATTERERS ARE PRESENT

The estimate formulas for the two-phase structure seminvariants (TPSSs ) in the presence of anomalous scattering are obtained from the estima te of the two-phase structure invariants [Hauptman (1982). Acta Cryst. A38, 632-641; Giacovazzo (1983). Acta Cryst. A39, 585-592] and the co nditional probability distribution of the TPSSs is derived from the ne ighborhood theory [Hauptman (1975). Acta Cryst. A31, 680-687]. A proce dure for estimating the one-phase structure seminvariants near O or pi (OPSSs) is proposed, based on the TPSS estimates. Test results for kn own structures with error-free data show accurate estimates for the TP SSs, even in the macromolecular case. For the OPSSs, the accuracy is r elated to the strength of anomalous scattering by heavy atoms in the m acromolecular case. The incorporation of the heavy-atom information im proves the results.