Hi,
My first thought was same with David: the truncation won't change the
crystal's space group. The symmetry of the crystal is reflected by the
symmetry of the amplitudes of many many reflections across all resolutions.
Ellipsoidal truncation itself only removes some very weak reflections from
the outer shells. The remaining reflections will still have a good number of
reflections carrying the symmetry of the crystal.
However a second thought on the anisotropic scaling and B-factor correction
led me to this scenario: suppose we have a crystal that's really P6, but we
have cowardly indexed it to a lower space group P2, with the 2-fold axis, b,
coinciding the real 6-fold axis. By losing the a=c restrain, the anisotropic
scaling along H and L now may not be strictly equal (for example, could be
caused by outliers that would have been identified and filtered out if
indexed correctly as P6), resulting in the loss of the 6-fold symmetry in
the scaled dataset. Apparently this is an artifact due to an improper SG
assignment before the anisotropic scaling and B-factor correction.
Just some crazy thoughts. Please correct me if I am wrong.
BTW, to Theresa: an very informative introduction on ellipsoidal truncation
and anisotropic scaling can be found here:
http://services.mbi.ucla.edu/anisoscale/
--------------------------------------------------
From: "Theresa Hsu" <theresah...@live.com>
Sent: Friday, April 27, 2012 3:18 PM
To: <CCP4BB@JISCMAIL.AC.UK>
Subject: [ccp4bb] Anisotropic diffraction
Dear crystallographers
A very basic question, for anisotropic diffraction, does data truncation
with ellipsoidal method change the symmetry? For example, if untruncated
data is space group P6, will truncated data index as P622 or P2?
Thank you.
Theresa
