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I just realized that this is an orthorhombic C222(1) space group. I didn't check it up but unless two of the cell-dimensions are nearly identical I think merohedral twinning is not possible for this space group, because the symmetry of the unit cell shape is not higher than the symmetry of the space group. Bart Eleanor Dodson wrote: > 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! >>> >>> 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. >>> >>> >> >> > > -- ============================================================================= Bart Hazes (Assistant Professor) Dept. of Medical Microbiology & Immunology University of Alberta 1-15 Medical Sciences Building Edmonton, Alberta Canada, T6G 2H7 phone: 1-780-492-0042 fax: 1-780-492-7521 =============================================================================