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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