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The Bruker-AXS twinning software was written by Robert A. Sparks. These
programs are designed to help the user sort out data sets that were collected
on samples exhibiting twinning of a non-merohedral nature. In this type
of twinning some or all of the points of the different reciprocal lattices
do not overlap. Merohedral twinning occurs when the two (or more) reciprocal
lattices exactly overlap. The following programs may be run individually
from a command prompt or may be run from the Gemini GUI in the Bruker
Software Package. The Gemini interface is recommended because of program
improvements and because of a nice outline of steps needed in a twin analysis.
There are four programs in the twinning suite:
- TWINDX produces one or more possible indexing solutions from a *.p4p
file.
- TWUTIL is a utility routine used to generate various *.p4p files.
- TWHKL generates a combined *.hkl (type 5) file from two or more *.hkl
files.
- TWROT determines the rotation needed to superimpose one lattice onto
another.
The programs may be called from the command line or may be accessed from
the SAINTPLUS group of programs. There are no command line qualifiers
for these commands. The programs do usually have reasonable values for
default parameters. The displayed (default) value for a parameter is accepted
by typing the slash "/" character followed by <enetr>. The usual
course of action is as follows: Threshhold many reflections in SMART.
Fill the reflection array if possible. Run TWINDX using the *.p4p file
created in SMART. TWINDX creates two files, a *.twx and a *.sum file.
The programs will use the *.twx file; people will find easier to read
the *.sum file. These two files contain essentially the same information
about the different indexing solutions. Look through the *.sum output
to select the best possible indexing solution(s). Run TWUTIL to calculate
the "complement" of the selected solution. Be sure to save the new *.p4p
file with a different root name. Run TWINDX with the new *.p4p file saving
the *.twx and *.sum with still a new root name. Look through the second
*.sum output to select the solution that has similar cell parameters to
the original solution. Once two separate solutions with similar cell parameters
but different orientation matrix components have been found proceed with
the next step. If no similar solution can be found, then select another
solution from the original *.sum file and start at step 4. Run TWUTIL
to generate a *.p4p file from the solution in step 3 but not the solution
from step 6. Run TWUTIL a second time to generate a *.p4p file from the
solution from step 6. Use these two *.p4p files in separate runs of SAINT
to integrate the data in the two reciprocal lattices. Correct the two
separate *.raw data sets for absorption generating two different *.hkl
files. Run the TWHKL program to merge the two *.hkl files into one new
*.hkl file with "HKLF 5" format. Run the TWROT program to determine the
nature of the twin rotation. Data sets that may look twinned on the data
frames (unreasonably close sets of spots in different parts of reciprocal
space) may actually be split rather than twinned. Split crystals usually
give one solution in step 2 above that uses nearly all of the spots; however,
integration in SAINT using this solution produces only a very poor data
set. When the three cell lengths of split crystals are relatively short,
it is sometimes possible to integrate the data using a wide spot shape
description.
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