RE: [ccp4bb]: a general question on anisotropic B-factors

["Ditlev Brodersen" <ding@mrc-lmb.cam.ac.uk>]

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Dear thiyaga,
 
  no, it's right that you can use that formula to describe the isotropic
B-factor as an approximate rmsd value for each atom, but there's no way
of converting an isotropic B-factor into an anisotropic one, let alone
calculate it from atomic coordinates alone. The information simply isn't
there. Atomic coordinates define merely infinitely small points in
space, whereas isotropic B-factors describe perfect spheres. To go from
there to an anisotropic ellipsoid, you have to have high-resolution data
and include all six components of the anisotropic B-factor matrix in the
refinement for each atom:
 
Uaniso = ((U11,U12,U13),(U21, U22, U23), (U31, U32, U33)), 
is a 3x3 symmetric matrix with U12=U21, U13=U31, and U23=U32.
 
For more information, refer to e.g. the SHELX manual.
 
  -- Ditlev

---

Ditlev E. Brodersen, Ph.D.
Postdoctoral fellow

Medical Research Council          Phone:  +44 (0) 1223 252945
Laboratory of Molecular Biology   Mobile: +44 (0) 7866 563050
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Cambridge CB2 2QH                 Email:
<mailto:ding@mrc-lmb.cam.ac.uk> ding@mrc-lmb.cam.ac.uk
United Kingdom                    WWW:
<http://alf1.mrc-lmb.cam.ac.uk/~ding>
http://alf1.mrc-lmb.cam.ac.uk/~ding


-----Original Message-----
From: owner-ccp4bb@dl.ac.uk [mailto:owner-ccp4bb@dl.ac.uk] On Behalf Of
S. Thiyagarajan
Sent: 07 April 2003 15:10
To: ccp4bb@dl.ac.uk
Subject: [ccp4bb]: a general question on anisotropic B-factors



given a set of atomic coordinates, can somebody tell me how the
anisotropic B-factors can be caculated.  if it is just the rmsd * (8/3)
* (pie**2) how are the directions assigned to the thermal ellipsoids
(e.g as in ORTEP)

regards

thiyaga



===================================================
== S. Thiyagarajan ==
== Research Scholar ==
== Department of Crystallography and Biophysics ==
== University of Madras, Guindy Campus ==
== Chennai - 600 025. India ==
=================================================== 




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