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I would be grateful for clarification on the intended and actual use of the 
_chemical_formula[] Core dictionary items. The dictionary entry includes

    _chemical_formula_ items specify the composition and chemical
   properties of the compound. The formula data items must agree
   with those that specify the density, unit-cell and Z values.

     _chemical_formula_iupac      '[Mo (C O)4 (C18 H33 P)2]'
     _chemical_formula_moiety     'C40 H66 Mo O4 P2'
     _chemical_formula_structural '((C O)4 (P (C6 H11)3)2)Mo'
     _chemical_formula_sum         'C40 H66 Mo O4 P2'
     _chemical_formula_weight      768.81


There are at least two different ways of creating such data items:
- transcription from a chemical logbook or paper where the formula 
represents the known chemical constitution. This can be derived from 
knowledge of preparation, analytical measurements, spectra etc.
- summation of the scattering matter determined by the diffraction experiment.

In many cases these will not agree. This can be due to disorder, unobserved 
atoms (e.g. H), etc.

It is important to be able to state the composition of the sample 
independently of the diffraction experiment. (Indeed if the structure is 
not solved this is probably all that can be stated!) Such compositions are 
extremely valuable in publishing crystallography to the chemical community 
and in internal checks of the CIF. However I gather that some software 
generates _chemical_formula fields directly from the _atom_site and 
_symmetry information.

I think it would be useful to have fields such as:
     _chemical_formula_weight _meas

     _chemical_formula_weight _calc

This would indicate that the first had been experimentally measured by some 
means other than diffraction and the second would show that they had been 
calculated fro the _atom_site + _symmetry information. The current
would give no information about the provenance of this information.


Peter Murray-Rust
Unilever Centre for Molecular Informatics
Chemistry Department, Cambridge University
Lensfield Road, CAMBRIDGE, CB2 1EW, UK
Tel: +44-1223-763069

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