Crystallography Online: the website of the International Union of Crystallography

Dear Howard,

Thank you for your careful reading through the proposed changes to the 
coreCIF dictionary.  Most of these are changes in dictionary wording and 
therefore do not affect the CIF coding.  Some of the wording is taken 
directly from earlier versions (e.g., the bit about absorption 
corrections) when our standards were not as strict as they should be.  
Thanks for catching these loose defintions.

The only place where you propose a definitive change is in item 8 and I 
accept this change.  The only alteration I would make to your propsed 
wording is to replace 'the same appropriate multiplicity' with 'a 
multiplicity of 1' which is in any case the default value.

I assume that with these changes you are prepared to approve the 
document?  I look forward to hearing from the rest of you.

David




Howard Flack wrote:

>
>------------------------------------------------------------------------
>
>
>
>Item 1.  _ATOM_SITES_SOLUTION
>  typo:  # _atomic_sites_solution_*
>  typo: "Dual space method" should be "Dual-space method"  
>  
> 
>Item 2. _CHEMICAL_ENANTIOEXCESS_
>  remove: "The composition of the crystal and bulk must be the same."
>          This is an evaluation criterion.  One can not impose on such measurements that they necessarily obey the results one hopes to obtain.  Since writing that sentence we have successfully done absolute-configuration determinations on bulk racemates that gave crystals twinned by inversion in various proportions.  CD measurements of the single crystals used for the diffraction experiment taken into solution provided the required measurement of enantioexcess of the crystals.
>  
>
>Item 3. DIFFRN_MEASURED_FRACTION 
>  The definition of _diffrn_reflns_theta_full contains as its first sentence: "The theta angle (in degrees) at which the measured reflection count is close to complete."  I suggest that the "close to complete" be augmented by a phrase expressing that this is taken to mean that the corresponding _measured_fraction be no less than 0.95 .
>   Idem for  _diffrn_reflns_resolution_full
>
>  Modification (twice):  Change "For space groups that do not contain a centre of symmetry the reflections h,k,l and -h,-k,-l are independent." to "For space groups that do not contain a centre of symmetry the reflections h,k,l and -h,-k,-l are independent for general values of h,k,l."
>
>  Why is the permitted range of _diffrn_measured_fraction_theta_full 0 to 1.0?  
>
>Item 4. _DIFFRN_STANDARDS_DECAY_%
>  Don't understand: "_enumeration_range          :100"
>  Why is the lower end of the range not -100 ?
>  
>  
>Item 5. _DISTRIBUTED_DENSITY          
>
>
>
>Item 6. EXPTL_ABSORPT_CORRECTION_T_max and T_min
>Typo: "The maximum and minimum transmission factors applied to the diffraction
>     pattern measured in this experiment.  "
>        should be
>       "The maximum and minimum transmission factors applied to the diffraction
>     intensities measured in this experiment.  "
>
>  "These factors are also referred to as the absorption correction A  or 1/A*." 
>   I wonder what this sentence means exactly. Is it defining A = T and A* = 1/T?  I thought the "absorption correction" was 1/T with A = 1/T and A* = T assuming, of course that one is correcting observed intensities and not the corresponding structure amplitudes!  Some systems incorporate the "absorption correction" into the model or calculated values.  The sense of the quoted sentence is entirely mysterious for me.  I wonder whether it is really necessary.  Moreover there is a problem with the English.  "These factors" presumably refers to the maximum and minimum transmission factors.  I'm sure that this was not the authors intention.
>   
>Typo: "As this value is the one".  Which "this" value is being referred to.  It is the same problem as in the sentence above.
>
>Typo: (twice) "specimen.  its value does not" should be "specimen.  Its value does not"
>
>Sort of typo: (twice) "The presence of this item does not imply that the structure factors have been corrected for absorption," should be "The presence of this item does not imply that the experimental intensities have been corrected for absorption,"
>
>
>Item 7  _GEOM_BOND_COUNT
>   OK
>
>
>Item 8. _GEOM_BOND_MULTIPLICITY
>"In cases where the 
>        full list of bonds is given, one of the series of equivalent bonds may
>        be assigned the appropriate multiplicity while the others are assigned
>        a value of 0."
>        
>    This is totally inacceptable.  It is also very confusing.  It even goes against CIF definitions (for the little that I know about them).  The ones with the zero assume that there is somewhere one and only one that does not have a value of zero. I suggest:
> "In cases where the full list of bonds is given, all members of a series of equivalent bonds must be assigned the same appropriate multiplicity."  
>
> 
>Item 9.  _GEOM_BOND_SITE_SYMMETRY
>  OK 
>
>
>Item 10. _PUBL_SECTION_KEYWORDS
>  Typo:  "enumerstion"
>
>
>Item 11. _REFINE_LS_F_CALC
>  OK
>
>
>Item 12. _SYMMETRY_EQUIV
>  OK    
>
>
>------------------------------------------------------------------------
>
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>


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