IS THE COMBINATION OF FAST, HIPPO, DREAM AND XD THE NEW WAY FORWARD TO CHARGE DENSITY ANALYSES? Bracke, Ben R. F. and Hursthouse, M. B. University of Wales Cardiff, Chemistry Department, P.C. Box 912, Cardiff CF1 3TB, UK.

The use of Area Detectors has become widespread in recent years and their applications both in X-ray and neutron diffraction show exciting results. An area in which its use to date has been far from routine so far is in the area of multipole refinement in order to obtain charge, spin and momentum density properties of the investigated compound.

The main problem until recently has been the way the area detector data are processed. Most commercial data collection packages provided with the diffractometer, including our FAST software, use an algorithm that assumes a priori knowledge of the peak shape which is in most cases based on an elliptical peak mask (Wilkinson et al., 1983; Kabsch, 1988a,1988b).

Recently, a new method of area-detector peak integration - the 'Seed-Skewness Method' (Bolotovsky et al., 1995) has been developed (and implemented in the program HIPPO) in which the integration is purely based on the statistical analysis of pixel intensities and hence uses no a priori knowledge of the peak. According to the first results mentioned in their article, it gives much better and more reliable results especially for the weak reflections, takes care of the (1/(2 separation and shows no significant difference in positional and thermal parameters after standard spherical atom refinements compared to data obtained via the earlier mentioned peak mask methods.

This observation was promising enough to start implementing this new Seed-Skewness method in our data collection procedure. By making use of the well established data reduction package DREAM (Blessing, 1986) and the recently developed multipole refinement and analyses package XD (Koritzantszky et al., 1994, we will present charge density analyses results on Creatine Monohydrate using both the Seed-Skewness method and the elliptical peak-mask method.

An interactive version of this abstract can be obtain from the URL: http://www.cf.ac.uk/uwcc/chemy/bracke/iucr/iucr96.html

Blessing, R. H., Crystallographic Rev., 1, 3-58 (1986).

Bolotovsky, R., White, M. A., Darovsky, A. & Coppens, P., J. Appl. Cryst., 28, 86-95(1995).

Koritsanszky, T., Hansen, N., Howard, S., Su, Z. & Mallinson, P., Berlin/Glasgow, first release: May 1994.

Kabsch, W. J., J. Appl. Cryst., 21, 916-924(1988a); J. Appl. Cryst., 21, 67-71(1988b).

Wilkinson, C., Khamis, H. W., Stanfield, R. F. D. & McIntyre, G. J., J. Appl. Cryst., 21, 471-478(1988).