Crystallography Online: the website of the International Union of Crystallography

Dear David:

    Many thanks for your message of December 23 and the accompanying draft
Working
Group Report. On checking the IUCr phase-identifier archive before replying,
it was a
surprise to find your item entitled "Discussion 6" in it. Most likely, that
item had been
misidentified as spam on arrival in my mailbox as had this message. I regret
the earlier
item was not noticed subsequently among the excessive quantity of spam now
received daily.

    May I start by suggesting WG members may find the July 2002 presentation
on
preliminary thinking about IChI to be of interest, as given at:


http://www.iupac.org/symposia/conferences/CIandXML_jul02/ICHI_Stein_jul2002.
pdf

although the summary in your Section 5 shows that considerable progress has
been made
since that conference. However, members may well wish to see further details
of the results
agreed upon during the IChI workshop at NIST in November 2003. Are these
expected to
become available soon?

      I am in full agreement with your proposal that a unique comprehensive
identifier for each
chemical compound be formed by adding the crystal phase identifier to the
IChI chemical
identifier. In reading your first draft, however, it is striking that no
mention is made of the
proposed method(s) of implementing such a system, possibly because they seem
obvious
to the specialist. However, a number of questions are likely to arise in
reading our Report
and I suggest it would be of value to our readers if it contained a section
that addressed
these and related issues so that our recommendations are set in their
fullest context.
These issues include the following: once a unique identifier system has been
agreed,
must it be reduced to a single algorithm to avoid the introduction of
variant identifiers? If
the latter is the case, then would it be advantageous to state or merely
refer to the algorithm?
Must each database adapt the algorithm to match its specific contents or is
that the
responsibility of the user? To the extent possible, the new section should
respond to
these and similar questions.

     More detailed comments follow:

     In the example of a material with a single crystal form, OsI3, I nopte
it is not listed in the
ICSD. Perhaps a better choice should be made?

    I agree with your proposal to add three crystallographic layers to the
four IChI chemical
layers. The choice between single and multiple letter codes depends upon the
answers given
to the questions above.

   I also agree with use of the space group number for layer 6 and, if
necessary, with the Wyckoff
multiplicity and letters in layer 7.

   I doubt if use of the Bravais symbol in the identifier would be of value.

   Addition of the comprehensive IChI identifier in a new field, probably
the leading field, in the CCN
phase nomenclature [see Acta Cryst. (2001). A57, 614-626 and Acta Cryst.
(1998). A54, 1028-1033)]
would be appropriate in database compilations. It would probably be
inappropriate elsewhere.

Happy New Year!

Sidney
------------------------------------------
Prof. S. C. Abrahams
Physics Department
Southern Oregon University
Ashland, OR 97520

Fax: (541) 552-6415    Tel. (541) 482-7942
Email: sca at mind.net
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