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Banning DIFABS (A Summary from circa 1st July 1994)
A discussion on whether the absorption correction software, DIFABS,
should be banned, was started on April 20th by Tom Spek on the newsgroup
sci.techniques.xtallography and seems
to have reached compatable conclusions.
It is hoped a agreed summary can be generated and submitted
to an appropriate journal.
The aims of a journal article would be to raise awareness
of both the potential applications of this newsgroup, and
the DIFABS issue.
There is concern over the use and misuse of absorption
correction software such as DIFABS to determine crystal structures.
Via discussion, this has resulted in the following actions and
conclusions :-
- DIFABS (and similiar software) is a legitimate package
in the crystallographer's 'back of tricks' - but users should
be made aware of its abuses and effects (e.g. software correction for
poorly installed and aligned equipment, inappropriate stages of the
refinement to be using the correction software).
- Raw data (not just corrected) should be provided with the
abstract in CIF format. The size of raw data files is not an
archiving problem given the size of modern tape and hard-disks.
- The DIFRAC software by Flack, Blanc and Schwarzenbach for
converting (possibly binary) diffractometer data from several commercial
diffractometers into CIF (or SCFS) format is available via anonymous
FTP at ftp.unige.ch:/pub/soft/crystal/DIFRAC (J. Appl. Cryst. (1992) 25, 455-
459). Thus there is now no excuse for raw (CIF) data not to be
submitted to the journal.
More emphasis, if possible, on "customers" providing journal
quality crystals for analysis.
Where this is not practical, data-collection strategies can be
altered to accomodate the crystal at hand. i.e.:-
- Psi-scans
- Time effective, accurate face-indexed measurements of the
crystal done off the diffractometer.
- Relatively cheap methods exist for obtaining computer (or
visual) images of the crystal on the goniometer for comparison with
computer generated drawings of the crystal.
- Other techniques [and order of preference?]
Use of such correction software should be explicitly stated
with the following submitted :-
[which are recommended - 1 or all?]
- "real" Fo and "corrected" Fo.
- Fc
- Fc^2
Methods for the easy, safe checking and alignment of single
crystal diffractometers are available and possible with even home
made equipment. For instance, using equipment such as cheap hand
lasers, it is potentally a trivial and safe job to check that the
X-ray beam is bathing the entire crystal. Routine checking of
equipment should be encouraged to minimize the collection of
poor quality data. Thus minimizing concerns about
the abuse of absorption correction software
to correct for systematic errors caused by poor
alignment and/or partial irradiation of the crystal.
A "round robin" comparing the effects of "various?"
absorption correction software vs uncorrected results would be
very beneficial.
Lachlan Cranswick - CSIRO _--_|\ lachlan@dmp.CSIRO.AU
Division of Mineral Products / \ tel +61 3 647 0367
PO Box 124, Port Melbourne \_.--._/ fax +61 3 646 3223
3207 AUSTRALIA
Crystallography World Wide Editor
- Acknowledgements