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Re: Draft changes to Core data names for multiplicity etc.
- To: Distribution list of the IUCr COMCIFS Core Dictionary Maintenance Group <coredmg@iucr.org>
- Subject: Re: Draft changes to Core data names for multiplicity etc.
- From: David Brown <idbrown@mcmaster.ca>
- Date: Thu, 17 May 2012 15:53:53 -0400
- In-Reply-To: <4FB4AFE3.4000901@shelx.uni-ac.gwdg.de>
- References: <20120516103234.GA13856@emerald.iucr.org><4FB4AFE3.4000901@shelx.uni-ac.gwdg.de>
I am glad that we have managed to get the multiplicity sorted
out in time for the new release of SHELX. If a program needs the
multiplicity it should, as George points out, be possible to
calculate this from the site symmetry order; all that is needed is
the multiplicity of the general position which can be found by
counting the number of symmetry elements that are almost always
included in the CIF. I am not sure that I see the problem with the disordered solvent. The occupancy that SHELX uses is correct, and the site symmetry order can be given for the actual site occupied. An atom close to a special position could be described as lying on the special position or not depending on the occupancy and the site symmetry order or multiplicity given in the CIF. Either choice would result in a consistent CIF. How a program handles this depends on the way it treats the symmetry. If it tests for special positions by seeing if any symmetry element generates the same initial coordinates within some rounding error there could be a problem if the author and reader of the CIF had different concepts of what a rounding error might be, but there is not much CIF can do about this. Adopting George's suggestion of assigning groups of atoms to a particular special position is intriguing but would need to be considered very carefully since it involves an entirely new approach to the way CIF describes chemical structures. It is not just a simple matter of extending existing definitions as George suggests. There is no problem in adding _reflns_Friedel_fraction_max and _reflns_Friedel_fraction_full to the dictionary. The change in the way in which the standard uncertainties are calculated is not a concern for CIF. CIF reports the standard uncertainties given by the author of the CIF and accepts no responsibility for their correctness, nor does it prescribe any particular method for calculating them. This change in SHELX procedure may be of interest to those who use the CIF, but it seems that in practice this change is small and unlikely to cause many problems. David Brown On 5/17/2012 3:59 AM, George Sheldrick wrote: Dear Brian et al., |
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- References:
- Draft changes to Core data names for multiplicity etc. (Brian McMahon)
- Re: Draft changes to Core data names for multiplicity etc. (George Sheldrick)
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