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*To*: Usha Nair <usha@nii.res.in>*Subject*: NCS worries*From*: "Hazes B." <bart@mrc-lmb.cam.ac.uk>*Date*: Tue, 29 Jun 1999 15:08:56 +0100 (BST)*Cc*: Lisa Edberg <edberg@onyx.cmc.uab.edu>, ccp4bb@dl.ac.uk*In-Reply-To*: <Pine.SGI.3.95.990629125042.13452C-100000@sbu.nii.ernet.in>*Reply-To*: "Hazes B." <bart@mrc-lmb.cam.ac.uk>*Sender*: owner-ccp4bb@dl.ac.uk

*** For details on how to be removed from this list visit the *** *** CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html *** If you have a 2, 3, 4, or 6-fold NCS operator then it is probably good practice to consider the possibility that it is actually crystallographic symmetry. A very high self rotation function peak is probably one thing to get you worried (but what is "very high"?). One requirement for crystallographic symmetry, unlike NCS, is that it should superimpose the crystal lattice onto itself. I have written a program that lets you try if this is the case. It is available over the WWW at http://perch.cimr.cam.ac.uk/forms/isitncs_form.html and just needs the unit cell parameters and the NCS operation as a 3x3 matrix. If the lattices clearly do not superimpose (small deviations normally occur due to small errors in the NCS operation and unit cell) then you have bona fide NCS. If they do superimpose then you either are dealing with crystallographic symmetry or some form of pseudo symmetry. I think it would be a great idea if data scaling/merging programs would provide a table with Rsym values for each independent symmetry operation (and one listing the statistics for just the Friedel mates). That might help people to spot pseudo symmetry or other anomalies at an earlier state. If it looks like you have NCS then it is also well worthwhile to calculate a "self-Patterson" using your observed intensities and all phases set to zero. If you have (pseudo) translational symmetry (this happens for instance when a NCS symmetry is parallel but not coincident with a crystallographic symmetry axis of the same order) it should give a distinct peak in this map. Bart

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