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Re: [ccp4bb]: anisotropic ellipsoids

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Not really. U11 U22 U33 U12 U13 and U23 are the tensor elements
in the chosen coordinate system (usually orthogonal). Only if
the coordinate system is rotated to the principal axes system
are U11 U22 U33 the displacements along the principal axes, in
which case U12 U13 and U23 are zero. U12 U13 and U23 only define
the orientation of the principal axes in the very loose sense that
their non-zero-ness indicates the deviation between the orthogonal
and principal axes systems.

Hmmm, quick flip through ortep manual, if that's what you're using. 
There seem to be some definitions in terms of orientations (which
I've never used) but not couched in terms of U elements ....



> According to many textbooks the first three of the thermal parameters
> U11 U22 U33 U12 U13 and U23 describe the displacements along the
> perpendicular principal axis of the ellipsoid and the latter three give
> the orientation of the principal axes with respect to the unit cell axes.
> However, I can't find anywhere how U12 U13 and U23 (apparently as
> direction cosini) exactly describe the orientation of the ellipsoid, say
> in a cartesian system.
> Any hint is appreciated (but don't suggest to try to follow the ortep
> code)...
> Thanks, Norbert


*                                                                        *
*                  Dr. Martyn Winn                                       *
*                                                                        *
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