Hi Frank,

I think it's about where you draw the line... It's very unlikely that at 3.5A the use of NCS will be a bad idea, and it's very likely that at 1.0A the use of NCS will be a good one. I don't know which side of that line the cases at around 2A resolution are (I believe the only systematic and careful study can answer this; or may be someone did it then I would appreciate a reference), and, as you rightfully pointed out - the automatic procedures are not perfect, so I usually try both -:)

Pavel.


On 11/26/2008 3:34 PM, Frank von Delft wrote: 

  
I am refining a structure with a resolution of 2.1 Angstroem using Phenix.
The spacegroup is P 31 2 1 (No. 152) with a twin fraction of 0.37 and twin 
law -h,-k,l.

In the refinement procedure the NCS (4 chains) constraints are on, using 
individual sites, ADP and occupancy refinement.
  
    
      
Just a suggestion to quickly try out... What happens if you turn off the 
NCS restraints and do some refinement without using NCS (with and/or 
without optimizing the weights)? I know it depends on many factors, but 
I've seen a good number of cases at around 2A resolution where the 
refinement was better if NCS is not used.
  
    
I can't not respond:  I've never had a case where *careful* evaluation 
and assignment of NCS groups did /not/ allow me to tighten NCS 
restraints (without screwing Rfree, of course).  But of course, it 
requires work:  the default procedures aren't good enough yet.

phx
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