FEWER STRUCTURE DETERMINATION ERRORS WITH THE SMART CCD SYSTEM. Robert A. Sparks and Charles F. Campana, Siemens Energy and Automation, Inc., Analytical Instrumentation, 6300 Enterprise Lane, Madison Wisconsin 53719-1173, USA.

Data collected with the new Siemens SMART CCD diffractometer systems not only yields better quality data than conventional scintillation counter diffractometers but also leads to fewer mistakes in structure determination. The default data collection strategy used for the collection of X-ray intensity data on the SMART system involves the collection of 1.3 hemispheres of data to 0.75 resolution (at 5 cm. crystal-to-detector distance) using 10 second exposures and frame widths of 0.3 in w. Although such a data set requires approximately six hours to complete, regardless of the size of the unit cell, the quality of data matches or exceeds that of conventional point-detector data sets collected over a much larger time periods (typically several days).

The above data collection strategy minimizes the choices which must be made by the user, thereby preventing mistakes. The determination of unit cell dimensions, reduced primitive cell, crystal system, Bravais lattice and Laue class may be done after the full data set has been collected and prior to integration. Typically, unit cell indexing is based upon 512 randomly chosen reflections and final unit cell parameters are refined from a basis set of 8192 reflections. The high sensitivity of the detector and the high degree of redundancy yields much more reliable data for evaluation of systematic absences and the determination of the correct space group. Because complete frames of data are saved, twinned and incommensurate crystals are more easily identified and in some cases structures can be determined from such crystals. Since the data set contains many redundant reflections, each with direction cosines, very good empirical or face-indexed absorption corrections may also be carried out without the need for supplemental data sets (e.g., psi scans).