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Re: [ccp4bb] twinned? |
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CCP4bb navigationCCP4bb <-- 2008 <-- April 2008 <-- 03 April 2008Subject: Re: twinned? From: Eleanor Dodson ccp4 {- at -} YSBL {- dot -} YORK {- dot -} AC {- dot -} UK Date: 2008-04-03 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. 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! >> >> The information transmitted in this electronic communication is >> intended only >> for the person or entity to whom it is addressed and may contain >> confidential >> and/or privileged material. Any review, retransmission, dissemination >> or other >> use of or taking of any action in reliance upon this information by >> persons or >> entities other than the intended recipient is prohibited. If you >> received this >> information in error, please contact the Compliance HelpLine at >> 800-856-1983 and >> properly dispose of this information. >> >> > > CCP4bb navigationCCP4bb <-- 2008 <-- April 2008 <-- 03 April 2008 |
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