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[ddlm-group] Finalizing DDLm

  • To: Group finalising DDLm and associated dictionaries <ddlm-group@iucr.org>
  • Subject: [ddlm-group] Finalizing DDLm
  • From: David Brown <idbrown@mcmaster.ca>
  • Date: Thu, 11 Mar 2010 10:35:02 -0500
Dear Colleagues,

I assume that we are essentially finished in resolving syntax problems, but in that discussion some items were identified as being related to DDLm rather than syntax, so before we settle into serious dictionary writing we need to understand the DDLm rules.

One item that I believe was raised under this heading was whether, if a loop contained a single set of items, it was necessary to formally include this in a loop structure.  If this is deemed to be necessary, then there has to be some way of identifying the items that must appear in a loop.  The presence in the dictionary of a _category_key.* item would seem to flag this, but it is applied at the level of the category rather than at the level of an individual item.  If the requirement that the loop structure must always be used, then all the items in the category must be loopable, i.e., the category cannot include items that would not normally be included in the loop, items for example that apply equally to all the listed items such as a scale factor that is the same for all the structure factors in a loop.  This seems to be workable, but I am not sure how the legacy CIFs would fit in, since categories may include some listable item and some non-listable items, and I am sure the listable items do not always appear in a loop if there is only one set of such items reported in the CIF.

Is this something that can be clarified fairly easily?  It has an important bearing on how the CIF dictionaries are written.

fn:I.David Brown
org:McMaster University;Brockhouse Institute for Materials Research
adr:;;King St. W;Hamilton;Ontario;L8S 4M1;Canada
title:Professor Emeritus
tel;work:+905 525 9140 x 24710
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