DATA Menu
Twinning Tools
This menu item deals with tools which allow the user to examine and process
data from twinned crystals
- ROTAX - written by Simon Parsons and Bob Gould of Edinburgh University.
This program requires that the structure has been solved and an FCF structure
factor file is available. It searches for possible twin-laws and gives a
figure of merit for each such possible (generally 2-fold axis) twin law.
- TwinRotMat - an option in PLATON. This does the same as ROTAX but uses
a different algorithm.
- Write HKLF5 File - for non-merohedral twins. Generally it is FAR
preferable to treat these at the data-reduction stage. However, such twins
may not be recognised early on, or it may be decided to continue with a single
lattice integration. This utility is an ad hoc method of
post-processing the reflection file. The important choice is to decide which
reflections will overlap their twin component, either fully, partially or not
at all. This can be done by consideration of three possible discriminator
functions :
- Delta function on index non-integrality
- angular difference between the scattering vectors
- difference between sin(theta)/lambda values
The method is highly empirical and it is necessary to try several
possibilities, refining model structures against the output HKLF5 files.
If a pair of reflections have their discriminator function (df)
less than parameter R1, they are treated as perfectly overlapped. If a pair
of reflections have df greater than parameter R2, they are treated as
non-overlapped. Between these two values they are partially overlapped and
ignored by the program (on the assumption that the intensities must be
seriously in error). Suitable twin law matrices may be read from the output files of
ROTAX or TwinRotMat or may be input manually. The routine writes a SHELX
format HKLF5 type file. See the SHELX manual for how to proceed from this
stage.