New Stratagies for the Refinement of Rotationally Twinned Specimens. Victor G. Young, Jr., Department of Chemistry, University of Minnesota, Minneapolis, MN 55455.

A new method will be introduced for the least-squares refinement of rotationally twinned specimens. The presence of rotational twins adversely affects the accurate intensity measurement of the primary reciprocal lattice in small-molecule crystallography. The secondary reciprocal lattice can interfere with the primary reciprocal lattice in two related ways. These interferences are either exact overlaps or partial overlaps of reflections from both reciprocal lattices as defined by the twin law. The former case is well understood. Popular least-squares refinement software usually has a provision for the refinement of exact overlaps with the addition of a single scale factor. However, the commonly accepted method for dealing with the systematic errors associated with partially overlapped reflections is to eliminate these data from the refinement altogether. This will be shown to be unnecessary for most situations. Partial overlaps can be refined in the same manner as exact overlaps after specific modifications have been made to the intensity data file. The addition of a number of scale factors to the least-squares refinement for a rotational twinned specimen will vary depending on the number of distinct partial overlap relationships. Several examples will be presented in the triclinic and monoclinic crystal systems to demonstrate this new method.