[buster-discuss] special position in refine v1.10.0

["F.Xavier Gomis-Rüth"]

Dear Busters,
Is there any way to introduce Ramachandran restraints for the refinement 
in cases of (very) low resolution?
And side chain rotamer restrictions to infavor energetically-favorable 
conformations?
Thanks,
Xavier

El 03/02/2012 18:40, Oliver Smart escribió:
> On Fri, 3 Feb 2012, Dale Tronrud wrote:
>
>>    I agree with Peitro.  I don't know of a reason why an atom must be exactly
>> on the symmetry axis.  Often the density on the axis is not spherical so either
>> you have anisotropic B's (for a small deviation) or the atom has to shift a
>> bit off center.
>>
>>    If a bad contact restraint is pushing the atom away, then the EXCLUDE
>> statement should do the trick.
>>
>> Dale Tronrud
>
> It can be a reasonable choice to model an atom or ion on a special
> position with a single atom that lies exactly on axis or position.
> I think that SHELX allows this to be done with constraints. I would
> like to allow BUSTER users to choose this with the distance restraints
> (next release).
>
> If the atom is allowed to drift from axis then it is a bit like allowing
> two alternates on a "normal" water. It is a perfectly reasonable modelling
> choice. But additional refinement parameters are introduced by allowing
> the atom to move off axis. In my limited experience density on symmetry
> axes is difficult to interpret and can have artefacts.
>
> The special position handling in BUSTER is not yet completely sorted. The
> current state-of-play is described:
>
> http://www.globalphasing.com/buster/manual/gelly/manual/gelly7.html
>
> The distance cut-off used does mean that drift can suddenly mean fully
> non-bonded terms appear and the atom flys away from the axis.
>
> Regards
>
> Oliver
>
> | Dr Oliver Smart                             |
> | Global Phasing Ltd., Cambridge UK           |
> | http://www.globalphasing.com/people/osmart/ |
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