anisotropy woes

Jiang Jiansheng jiang at LAPLACE.CSB.YALE.EDU
Fri Jul 7 10:21:04 EST 1995


>   I've just started refining something against some disgustingly
> anisotropic data.  The manual shows how to apply an overall anisotropic
> B to Fobs, which is fine in theory, but in practice sigma is ignored in
> refinement, so I would rather not scale up the reflections in the bad
> direction where F/sigma is very low to be the equals of the strong
> reflections in the good direction.  I tried to get around this by
> refining an anisotropic overall B directly before refining the model,
> but it appears that only the trace of the matrix is saved, so as soon
> as Fcalc is updated, all benefit is lost.  Does anyone have a better
> idea?  Should I just alter Fobs and add some sigma-based weighting
> scheme?  Or should I just close my eyes, alter Fobs, and proceed as
> usual?
>
> Thanks in advance,
> Phoebe Rice
> Phoebe at vger.niddk.nih.gov

It is recommended to alter Fobs after the anisotropic overall B refinement.
The reason is that we only save the trace of the matrix to the B factor array
and we do not store the whole B tensor (anisotropic B parameters) in the
coordinate file (may be in the future).

Definitely one could apply the sigma-weighted scheme to Fobs
for the overall anisotropic B refinement (but may need some 

tricky operations prior to version 3.1).

Jiansheng Jiang.



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