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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