It is not really possible to detect twinning by the simple moment and
cumulative distribution tests for data from a crystal with pseudo
translation. As Bart says, twinning decreases the value of the moments,
whilst pseudo-translation increases them, so the two effects tend to
cancel out.
There is a reference to the L test: J. Padilla & T. O. Yeates. A
statistic for local intensity differences: robustness to anisotropy and
pseudo-centering and utility for detecting twinning. /Acta Crystallogr./
*D59*, 1124-30, 2003.
<http://scripts.iucr.org/cgi-bin/paper?S0907444903007947>S
They suggest using neighbouring reflections pairs to test . This can
often overcome the problem associated with pseudo-translation. However
it is quite sensitive to data quality.
See http://nihserver.mbi.ucla.edu/pystats/
Eleanor
Bart Hazes wrote:
Hi Qiang,
A normal data set has a unimodal intensity distribution with a
predictable shape. When there is twinning the distribution remains
unimodal but becomes sharper and this is picked up in the twinning
analysis. When there is pseudo-translational symmetry, as you indicate
you have, then the intensity distribution becomes bimodal with one set
of reflections systematically strengthened and another systematically
weakened. This makes the whole distribution broader, just the opposite
of what twinning does, and therefore shows up as "negative twinning"
in the analysis.
Bart
Qiang Chen wrote:
Hi all,
The data I am working on has a strong translation vector. The space
group
is C2221 and resolution is 2.3 angstrom. There are two molecules per AU
with a pseudo-2-fold axis.
On the cumulative intensity distribution plot, the theor and obser
curves
totally do not overlap. I did "detect_twinning" from CNS, and there
is the
result:
<|I|^2>/(<|I|>)^2 = 3.2236 (2.0 for untwinned, 1.5 for twinned)
(<|F|>)^2/<|F|^2> = 0.6937 (0.785 for untwinned, 0.865 for twinned)
Does the result mean my data is not twinned?
Any suggestion will be highly appreciated.
Thank you!
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