DELABS - DELTA-F Based Correction

ABSORPTION CORRECTION TOOLS

PLATON HOMEPAGE

The DELABS filter in PLATON for empirical absorption correction requires two inputfiles:

1. shelxl.hkl : (SHELX HKLF 4 + Direction Cosines) reflection file
2. shelxl.res : A file containing the current set of refined parameters.

The calculations may be invoked with:

platon shelxl.res

followed by clicking on the DELrefABS button on the main PLATON option menu.

or the interactive keyboard instruction

CALC DELABS (NOCHECK)

Notes:

• The required direction cosines on shelxl.hkl should conform with the SHELX76 style. Such a file should be made available by the data reduction program used.

• The optional NOCHECK sub-keyword overrules the checking of the direction cosines. This option should be used for good reasons and with great care.

• DELABS will calculate the structure factors needed for this procedure internally using the positional and displacement parameters given in the '.res' file. When applicable, all anisotropic displacement parameters are converted to U(eq)'s.

• DELABS can also handle (pseudo)-merohedral twins described with the SHELXL TWIN/BASF mechanism.

• A DELABS run may be done (when applicable) on the reflection file modified by SQUEEZE.

• Be aware of the fact that the use of DELABS/DIFABS as an Empirical Absorption correction technique is considered to be 'sloppy crystallography' by some crystallographers/referees/journals.

• DELABS/DIFABS will in general 'correct' for more than just absorption. It will take care of systematic errors related to data collection on crystals too large for the homogeneous part of the X-Ray beam.

• In general, anisotropic thermal parameters will be less meaningfull because their average deviation from spherical tends to be 'absorbed' in the correction, making the heaviest atom in the structure look isotropic.

• This implementation attempts to avoid possible human errors due to (or related with) cell transformation matrices, scaling factors, anisotropic displacement parameter model etc.

• See also Discussion on DIFABS and Louis Farrugia's comparative absorption test

EXAMPLE DATA

ABSORPTION CORRECTION TOOLS

PLATON HOMEPAGE