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Adding Restraints for PDF Fittings
DanielIrving #1
Member since Dec 2018 · 12 posts · Location: Southampton
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Subject: Adding Restraints for PDF Fittings
Hi everyone again,

I have another question that I'm unsure of and can't really find much information on currently so I'm hoping someone may know more about this. I am attempting to fit some PDF data of a zeolite to a CIF file (generated from a DFT lowest energy calculation), and I am trying to add restraints to this. I have been allowing the atomic positions to refine, which unsurprisingly, gives a good fit but an incredibly unlikely crystal structure.

I have looked at a few tutorials and forum posts that use the "Distance_Restrain" macro, and I have attempted to incorporate this into the calculation but the experimental fit appears to simply flat line. I am unsure to whether this is due to the way I have set up the restraint or whether that this particular restraint is only useful for structure solution. If the latter, is there any other way to apply this kind of restraint for PDF.

Sorry for the length of this post, but any help on the matter would be greatly appreciated!

Daniel
johnsoevans (Administrator) #2
User title: John Evans
Member since Aug 2009 · 219 posts
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Daniel,

The Distance_Restrain macro should work just the same way. 

The most common problem is not specifying the correct pair of bonded atoms.  Before you put in the restraint do an append_bond_lengths calculation.  Then specify the atoms using their full labels (e.g. Si1 0 0 0 0 0 O2 0 0 1 0).  Before you try refining, run with "iters 0" and see what bond lengths get reported in the macro.  If they're not what you're expecting then you've got a problem, and the refinement is liable to diverge.
DanielIrving #3
Member since Dec 2018 · 12 posts · Location: Southampton
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That worked perfectly, the output CIF is no longer blowing up! Further to this, I was wondering if there was a way to add generic restraints e.g. all P-O-Al angles being around 145 degrees (+/-5 degrees), or do you have to set up individual restraints for each unique atom?

Thanks again!
johnsoevans (Administrator) #4
User title: John Evans
Member since Aug 2009 · 219 posts
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Individual angles is probably the easiest/safest way.  You might be able to do something more general based on P-Al distances but I personally wouldn't risk it.
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