Conversion between B's and U's

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Hi all,

I've just realised that the mmCIF provides no formal specification of the 
conversion factor between B's and U's. If a program module looks for 
_atom_site.B_iso_or_equiv, say, and it is absent, it can determine from 
the ITEM_RELATED DDL category that it is related to 
_atom_site.U_iso_or_equiv by a constant, but has no way of determining 
what that constant is. 

In all the time that I have been looking at the dictionary, I had just
assumed that when I got around to it, I would be able to write a routine
that would return a B-value, when a U is present in the CIF, and that I 
wouldn't have to hard-code crystallographic knowledge into my software. 
Just goes to show how important it is to check everything.....

The only way around this, without changing the DDL, is to specify 
different unit names for B's and U's, and give the conversion factor in 
the ITEM_UNIS_CONVERSION category.

Cheers,
Peter.

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Peter Keller.            \ "We kill the cows to make jackets out of 
Dept. of Biology and      \  them, and then we kill each other for the
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