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CoreCIF revision 2.3



2003-06-25

Dear Colleagues,

     DEADLINE FOR RESPONSE: JULY 4.  PLEASE SEE THE NOTE BELOW
ABOUT THE NEED TO MEET THE PUBLICATION DEADLINE FOR INTERNATIONAL
TABLES VOLUME G.

     The copy deadline for International Tables Vol G is
approaching, and Brian McMahon has suggested that it would be
advantageous to include the current round of Core revisions
(2.3).  In order to meet the deadline, the revisions would have
to be fully approved by COMCIFS by the end of July.  This means
some quick work on our part, and deferring some of the more
complex revisions to version 2.4 (which we could continue to work
on at our own speed).  There are however a couple of minor items
that we can probably approve in the next few days and include in
version 2.3.  This would give us (that is, the coreDMG) 10 days
or so to review all the revisions so far, allowing us to get a
final approval from COMCIFS in time to meet the publication
deadline.

     I propose the following timetable:

June 25   Two minor proposals circulated to the DMG (they are
          included below, so we are meeting the first deadline).

July 4    DMG approval of these two proposals.

July 7    Posting of the cumulative list of the 2.3 revisions so
          far reviewed by the CoreDMG with a request for final
          CoreDMG approval by July 17.

July 18   Revision 2.3 passed on to COMCIFS for final approval.

July 31   Receipt of final COMCIFS approval.  The Core Dictionary
          2.3 passed to IT for publication.

In order to meet this timetable, it will be necessary to drop an
item if an issue is identified during any of the reviews since
there will not be time to resolve the issue through discussion.
However, editorial changes, which include clarifying the
definitions, can be made at any time during the approval process
without prejudicing the timetable.

     There are advantages in having as many of the revised items
as possible appear in ITG where they would be more readily
accessible.  In particular the items requested by Acta Cryst. and
the CCDC are likely to be well used, and it is also desirable to
have the space group items in the core, as they are basic to the
reporting of symmetry.

     There are several items on the list I originally circulated
that we have not yet discussed.  These will have to be held over
to version 2.4 because we do not have time to approve them before
the deadline.  They include:

1. Descriptions of occupational and displacive disorder.
          This was my suggestion to rationalize the way in which we
     handle disorder, but I have not yet thought it through.  The
     problem may be intractable, but is worth exploring.

2. Twins.
          Simon Parsons and colleagues are working on this.  Their
     proposal is not yet ready to bring to the DMG.

3. Machine independent description of the orientation of the
crystal.
          The present orientation matrix is unsatisfactory because it
     is undefined and depends on the diffractometer being used.
     The new definition could probably be based on the
     definitions found in imgCIF. I do not believe this item is
     often used and I am not aware of an urgent need. It
     obviously needs careful thought.

4. Details of scans for individual reflections (angles, ranges
etc.).
          Suggested by Curt Haltiwanger.  These will require a new
     category diffrn_refln_scan.  Curt is currently preparing
     definitions.

5. List of peaks in the electron density.
          Currently we can give the maximum and minimum in the
     difference electron density.  There was a request for a loop
     that would permit all the peaks in the electron density to
     be listed.

6. The ability to describe several different diffraction
experiments in the same data block.
          This would allow reporting of all the experiments used in a
     simultaneous refinement (e.g. where both x-ray and neutron
     intensities were used).

     There were no comments on the items in Discussion List 6 so
these have been added to the cumulative list of revisions for
version 2.3.  Two new items are proposed in the present list
(List Number 7).  Any comments should be posted on the CoreDMG
list by July 4.  If I receive no objections by that time, these
items will be added to the cumulative list of revisions and
posted for final approval by the DMG according to the timetable
above.

#######################################
#
#    NEW ITEMS
#
#######################################
#
#     DIFFRN_REFLN
#        _diffrn_refln_status
#     GEOM
#        _geom_bond_multiplicity
#
#######################################
#
#######################################
#
#  DIFFRN REFLN
#
#######################################

data_diffrn_refln_status
    _name               '_diffrn_refln_status'
    _category           diffrn_refln
    _type               char
    _list               yes
    _list_reference     '_diffrn_refln_index_'
   loop_
    _enumeration
    _enumeration_detail
      incl
           'Reflection expected to have non-zero intensity'
      sysabs
           'Reflection considered to be systematically absent'
    _example                       ?
    _definition
;       A flag indicating whether a reflection would be
        systematically absent in the space group assumed during
        the measurement of the diffraction intensities.
;
# COMMENT:  The purpose of this item is to allow reflections that
# are believed to be systematically absent to be flagged.
# Although the assumption of a particular space group belongs
# properly to the model assumed as the basis of refinement, a
# space group is usually assumed early in the measurement
# process and is used to direct the measurement strategy.  There
# are occasions when some or all the assumed systematic absences
# are measured in order to confirm the space group.  This flag
# allows them to be identified either for closer examination, or
# for exclusion from the reflection count.
#
# Are there other values that should be added to the enumeration
# list?
#
# STATUS: Open for discussion.
#
##########################################
#
#      GEOM_BOND
#
##########################################

data_geom_bond_multiplicity
    _name               '_geom_bond_multiplicity'
    _category            geom_bond
    _type                numb
    _type_conditions
    _list                yes
    _list_reference      '_geom_bond_atom_site_label_'
    _enumeration_range   0:
    _definition
;       The number of times the given bond appears in the
        environment of the atom listed in
        _geom_bond_atom_site_label_1.
;
# COMMENT: In high symmetry structures when many bonds are
# related by symmetry, it is not necessary to list all the bonds
# in the environment of the first named atom.  Some users may
# wish to give only the symmetry independent distances and a
# multiplier to indicate how many such bonds are found in the
# atomic environment.
#
# STATUS: Open for discussion

     Please reply with comments or approval to the coreDMG list
before July 4, by using the 'reply to' option of this message

               Best wishes,

                    David


*****************************************************
Dr.I.David Brown,  Professor Emeritus
Brockhouse Institute for Materials Research,
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773
idbrown@mcmaster.ca
*****************************************************


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