Crystallography Online: the website of the International Union of Crystallography

Howard,

I take your point about the bond multiplicities, but even with your 
attractive solution there are two problems raised by the CIF rules:

1. A given dataname may only appear once in a datablock.

2. One cannot legislate in CIF when items may not appear in a loop 
because other items are present i.e., you cannot have a rule: if A is 
included, B is forbidden and vice versa.

The only solution is to define a new category: geom_bond_unique, that 
would be used when only the symmetry unique bonds are to be listed (with 
or without multiplicities).  It will require developing definitions for 
the other three items in the list below.

This loop belongs to the geom_bond_unique category.  The category 
geom_bond then remains unchanged from the way it is at the moment.

N.B. _geom_bond_unique_multiplicity is define only in this new category.   _geom_bond_site_symmetry_1 and _*_2 are defined only in the current geom_bond category.  This way they cannot both appear in the same list.


loop_
_geom_bond_unique_atom_site_label_1
_geom_bond_unique_atom_site_label_2
_geom_bond_unique_distance
_geom_bond_unique_multiplicity
Ca1   F1   2.495(9)    1
Ca1   F2   2.291(10)   2
Ca1   F2   2.391(11)   2
Ca1   F3   2.214(11)   2
Cr1   F1   1.940(11)   2
Cr1   F2   1.918(9)    2
Cr1   F3   1.848(10)   2

The second example you gave (which is the same as the loop in current 
use) is unchanged.

If this meets your requirements, I will draft an amendment to the 
circulating document when I return from ECM24.

David
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