CCD data and shelxl-93 refinement

gsheldr at shelx.uni-ac.gwdg.de gsheldr at shelx.uni-ac.gwdg.de
Thu Dec 12 02:36:56 EST 1996


Reply to message from Xiang Ouyang "CCD data and shelxl-93 refinement" 

Dear Dr Ouyang

I strongly recommend processing Siemens CCD data with the program called
SADABS before refining with SHELXL-93.  SADABS not only applies an
empirical absorption correction, it also puts the sigma(I) values onto
an approximately absolute scale.  It is avaliable free to Siemens CCD users
only.  If the crystal is triclinic it is important to make several different
phi or omega scans to get enough redundancy for SADABS to work well, for high
symmetry crystals the standard scans are adequate, so it should work well on
your data (however it must be told the correct Laue group, probably 6/mmm).

You cannot fudge the bad sigmas with the WGHT card in SHELXL-93,  With HKLF
you can multiply the sigmas by a constant, but this is unlikely to be a good
approach.

P6cc may well be the right space group, though there is a possibility of
merohedral twinning in such cases (what are the mean E^2-1 value and the
Rint values for the various possible Laue groups ?).

Sincerely, George Sheldrick





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