SHELXA

4.34 - SHELXA - Empirical Absorption Correction


Synopsis
This program applies "absorption corrections" by fitting the observed to the calculated intensities as in the program DIFABS. SHELXA is intended for EMERGENCY USE ONLY, eg. when the world's only crystal falls off the diffractometer before there is time to make proper absorption corrections by indexing crystal faces or by determining an absorption surface experimentally by measuring equivalent reflections at different azimuthal angles etc.

The following restrictions apply to the use of SHELXA:

The absorption is modeled by spherical harmonic functions using full-matrix least-squares more or less by the method of Blessing (1995). It is not this model that should be regarded as dubious, just the way SHELXA misuses it. Data are used for parameter determination if the I/(I) ratios for both the observed and calculated intensities exceed a given or assumed threshold (equal to 5.0). The delta(U)/lambda**2 parameter specifies an artificial U/2 value that is applied to the calculated intensities. This helps to prevent atoms going NPD, but the default value is zero. The highest even and odd order spherical harmonics to employ must be specified. The refinement could be unstable if these are too high, especially if only part of reciprocal space is sampled, eg. because only an asymmetric unit was collected for a high symmetry structure. Allowed values are (0,2,4,6,8) and (0,1,3,5,7) respectively.
The data may be re-processed when, for example, extra atoms are added. However, as with DIFABS, best results are obtained if the procedure is last run with the final ISOTROPIC model. Re-running it after anisotropic refinement will result in a deterioration of the structure and (most important) the R-factors. The U fudge should not be used repetitively, because the effects will be cumulative !
Note that all esd's estimated by SHELXL using data "corrected" in this way will be invalid unless the number of parameters used in the absorption model is input as the third L.S. parameter. This number depends on the number of even/odd harmonics used and is output by SHELXA. The program can read either standard SHELX direction cosines (relative to the crystal reciprocal axes), or orthogonal direction cosines calculated by the method given in Blessing's paper.


Files read
  1. name.FCF file written by SHELXL-97 (using LIST 4 or LIST 6 and any combination of MERG, OMIT etc). A SHELXL-93 .FCF file is NOT suitable because some information is missing.
  2. name.HKL - uncorrected reflection file written by programs XCAD4 or PROFIT. Direction cosines are required, which MUST correspond to the unit cell in the name.INS file.

Files written
  1. SHELXA.HKL - absorption corrected reflection file
  2. SHELXA.LST - List file giving summary of the applied corrections
  3. SHELXA.CIF - CIF containing relevant information