Re: Draft CIF twin dictionary for approval

Dear David,

I looked over the twin CIF definition document.  To be honest, at first reading the definitions needed and used to handle reflection data seem very complicated for general use.  Below are some thoughts. I don't know if these are addressed in the current effort or things not yet quite resolved.  Maybe they are, but I did not get my head around all this sufficiently well to know.

First, there are some typos I detected.
"the this" appears in several places and probably "this" has to be deleted.
In a description, the range 620 - 624 is given followed by 624 - 629. Probably the latter starts at 625.

Would the definitions for reflection data allow the full input reflection file to be reconstructed, e.g. for use in CRYSTALS, SHELXL, etc.?  I think the answer is yes, but I want to be sure people agree.

A fully overlapping reflection would have n individual contributors, each with its own values of h,k,l, but one cannot know the contribution of each to the measured F**2 and sigma (in the input data).  So how is that handled in the current definitions; i.e. I cannot see an example where there are two entries with different individual_id and h,k,l, but the same F_squared_meas and F_squared_sigma.  I am guessing that the _twin_contribution_ list in combination with the _twin_reflection_ list achieves this.  OK, but then you have two lists which must be given together, so things are getting complicated and long.  Furthermore, the appearance of all h,k,l assignments in the _twin_contribution_ list is then to some extent duplicated (and complicated) by the use of h,k,l again, but only from one component in the _twin_reflection_ list.  Understanding how these two lists are constructed, indexed and combined seems rather complicated and takes quite a bit of digesting of the dictionary to come to grips with.  Could it be simpler???

Compare this with lines in the HKLF5 style input for SHELXL (CRYSTALS is similar)...
   h   k   l  Fo**2     sigma  component (=individual_id)
  -4   2  -5  440232    6723  -2
  -4  -1   6  440232    6723   1
  -3  -1   6  336093    5357   1
  -2  -1   6 2138204   47562   1
  -1  -1   6   71870    1617   1
   0   1  -6 2044486   44513  -2
   0  -1   6 2044486   44513   1
The first and last two lines are overlapping refls from both components (common F**2 and sigmas), lines 3-5 are non-overlaps from component 1 only.

I am not trying to suggest we need to follow the SHELXL path always, but I am interested in keeping things relatively simple and easy to understand for users.  To me, the SHELXL idea seems less complicated than two separate _twin_contribution_ and _twin_reflection_ lists.

Just my thoughts...

P.S. Following our discussions in Warwick, I was working on a list of CIF items that we might think about updating or other new things needed.  I will get that to you eventually, but things have been too hectic lately for me to devote much time to it (which is why I could not accept James Hester's kind invitation to step into your shoes).

Best wishes,
Tony

Dear Colleagues,

After many years of gestation, a draft CIF dictionary of items for describing twinning in crystals is now available.  It is attached to this email which is being circulated to the core CIF Dictionary Management Group for your approval, this being the final step in the formal COMCIFS approval process.  If you are receiving this message you are invited to review the attached draft and either indicate your approval, or draw attention to potential problems, by replying to the core DMG list at <mailto:coredmg@iucr.org>coredmg@iucr.org. When approved the twinning dictionary will become an addendum to the coreCIF dictionary.

The draft is open for review for six weeks ending  on 11 November 2013.  If you have not replied by then, it will be assumed that you approve of the attached document as circulated.  If any questions are raised we will try if possible to resolve them within the review period.

The dictionary is also available at the URL <https://github.com/jamesrhester/twinning-dic>https://github.com/jamesrhester/twinning-dic

I look forward to receiving your response.

David Brown
Chair of the core CIF Dictionary Management Group

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