Re: [magCIF] magCIF working group

Dear magCIF working group members (and others),

Our first big decision is whether to form a separate magCIF dictionary or to propose additions to existing dictionaries ("core", "symmetry" and "modulated structures").

Advantages of a separate dictionary:  (1) This would be the fastest route to approval.  (2) The discussion would be influenced primarily by those with experience with magnetic structures.

Disadvantages of a separate dictionary:  (1) Nearly all of the magnetic tags are highly analogous to tags in other dictionaries.  Separating them in a separate dictionary weakens the analogies.  (2) We have to decide on a specific version of the Dictionary Definition Language (DDL) for creating the dictionary.  Because the core and modulated-structures are written in the older DDL1, while the symmetry dictionary is written in newer DDL2, either choice will weaken the connection to a large number of analogous tags.  There is also an even newer DDL version called "DDLm".

Advantages of adding to existing dictionaries:  (1) The obvious analogies between magnetic and non-magnetic tags are emphasized.  (2) No decision between different DDL versions is necessary -- each existing dictionary is already version specific.

Disadvantages of adding to existing dictionaries:  (1) Different committees would study and vote on tags proposed for different dictionaries.  (2) Those debating the additions to each dictionary might have little knowledge of the unique issues surrounding magnetic structures.  (3) The process of approval could take a long time to complete, and requested changes could create incompatibilities between tags submitted to different dictionaries.

I think that adding to existing dictionaries would produce the best final outcome if time were not an issue.  But the current adoption of various magnetic-CIF variants by different software packages makes the matter very time sensitive.  For this reason, I propose that we proceed to develop a separate magCIF dictionary and to move forward as quickly as possible.  Please share your opinions.

If you agree to create a separate magCIF dictionary, then we must deal the matter of the DDL version.  While the CIF tags developed for various DDL versions seem similar on the surface, the technical differences are quite profound.  I personally take a superficial (practical) view of the whole matter -- I am primarily annoyed that the DDL2 tags always use "." to separate the "category" from the "item", whereas DDL1 tags always use "_" to accomplish this.  Whichever decision we make, we will place analogous tags with a mixture of "." and "_" separators side by side (sometimes in the same loop) in a CIF.  This problem already exists for users of the symmetry dictionary.  But it will be much worse for magCIF tags due to the strong analogies between magnetic and non-magnetic tags within the same category and sub-category.  I imagine that some software authors will ignore whatever standard we choose, and simply use either "." or "_" uniformly.  And as a result, many programs will
 need to take the approach of accepting either "_" or "." as input, and always writing both "_" and "." in the output (two copies of each tag).  I believe that there is at least one major structure-analysis program that already does this for symmetry-dictionary tags.  If I misunderstand this whole situation, please correct me.

COMCIFs is quite interested in our decision on the separate-dictionary issue and the DDL-version issue, but will probably have less input on the real meat of our work, which is the structure (name, value ranges, units, and description) of each tag.  I propose that develop the tag structures now, in a simple ascii-text file that includes a primitive markup system, and then convert it into whatever DDL version is decided upon at a later date.  Please share your opinions.

Best wishes,
Branton

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Branton J. Campbell, Professor
Department of Physics & Astronomy
Brigham Young University
N261 ESC, BYU, Provo, UT 84602
Tel: 801-422-5758 Fax: 801-422-0553
Email: branton_campbell@byu.edu
http://www.physics.byu.edu/faculty/campbell/

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