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You need: a) to have an image of your crystal mounted on the goniometer in orientation, in which the axis of interest (6-fold) is perpendicular to the beam. You should know the system well enough to relate the coordinate system of the crystal on this image to the beam-gravity coordinate system. b) you collect in this orientation diffracion image(s) sufficient for indexing c) index the lattice and chose hexagonal lattice and refine parameters d) then you collect data set from the crystal so you can solve the structure in this coordinate system. Because space group is polar, there are always two non-equivalent choices for the coordinate system and you cannot replace one with another. The critical step is performed after (c) and depends on software -- I describe here HKL2000. On the image oriented according to (a) check where are the positive and negative values of hexagonal index (l index - use zoom window). HKL2000 present the diffraction image of the detector from the perspective of an incident beam. Therefore, you would need to correlate positive and negative values of indices with the optical image coordinate system from (a). Remember that optical axis (of the camera imaging a crystal) may not be along the beam. The structure solution will provide you with the direction of axis c. D. > I'm interested in knowing how to figure out the relationship between the unit cell contents and the crystal habit in these crystals (small attachment, two roughly orthogonal views). > Space group is P64 (enantiomeric) , and you can clearly see the > six-fold. The question becomes how to determine which direction the screw axis is going with respect to "top" and the "base" of the > pyramidal crystals (right image) so I can gauge how/why the crystals grow this way based on the cell contents. > Thanks in advance. > --paul Dominika Borek, Ph.D. *** UT Southwestern Medical Center 5323 Harry Hines Blvd. *** Dallas, TX 75390-8816 214-645-9577 (phone) *** 214-645-6353 (fax)