USING teXsan TO STUDY CRYSTALLOGRAPHY Beverly R. Vincent, Molecular Structure Corporation, The Woodlands, TX

Automated instrument control and crystal structure analysis software have made the determination of a crystal structure faster and easier than ever before. However, such software has allowed users to perform crystallographic analyses without requiring them to understand the fundamentals involved. With our small-molecule structure analysis package, teXsan, we have incorporated utilities which may be used to learn more about the diffraction experiment and to see it in terms of classical techniques.

Interactive graphical displays make use of the measured intensities to simulate Weissenberg, rotation and precession film experiments. The final coordinates are used to simulate the results of a powder diffraction experiment. Other graphical programs are provided to allow the user to see the results of the analysis in terms of space group symmetry. The user is able to generate packing diagrams which include the symmetry elements as designated in International Tables. These drawings, combined with a simulated precession or Weissenberg photograph, may be used to investigate the effects of space group symmetry on the diffracted intensities, including systematic absences.

Each automated procedure in teXsan is accompanied by extensive output which shows how and why a certain decision was made, as in the space group selection routine. An automatic structure solution and refinement routine is accompanied by a descriptive discussion of each step of the procedure, serving as a potential model for the less automatic procedures required for difficult structure analyses. Utilities are provided for analyzing and understanding the finished crystal structure.