DIFABS-CAD4

4.31 - DIFABS absorption correction (CAD4 version)

Synopsis
A low order term Fourier series is used to model an absorption surface for differences between the observed structure factors and those obtained from a structure factor calculation (after isotropic least squares refinement). Spherical polar angles are used to define the incident and diffracted beam path directions. This reduces the number of angles required to four - viz. PHI(p), MU(p), PHI(s) and MU(s). A theta-dependent correction is evaluated to allow for diffracted beams with different path lengths occurring at the same polar angles Again, a low order term Fourier series is used; in Bragg angle, THETA.

DIFABS is written by Stuart and Walker (Acta Cryst (1983), A39, 158). This version is specific for CAD4 geometry and usually provides a more satisfactory fit than the general version. It should be used if CAD4 data files are available. Please also read the misuse of DIFABS

Files read
  1. name.OMX - the CAD4 orientation matrix (written by programs XCAD4 or PROFIT).
  2. name.INS (for unit cell information only).
  3. name.FCF - a structure factor file written by SHELXL using the LIST 4, MERG 0 instructions. IT IS VITAL that this file is prepared with a model where ALL the atoms are refined with ISOTROPIC thermal parameters, and NO extinction correction has been made. It is desirable to obtain the best isotropic model, including H-atoms if possible.
Files written
  1. DIFABS_CAD4.HKL - absorption corrected reflection file
  2. DIFABS.LST - List file giving summary of the applied corrections
  3. DIFABS.CIF - CIF containing relevant information