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Re: Draft and analysis of proposed change to DDL1.4 tofix _atom_site_aniso_label

I have been reading this correspondence with interest (as have other 
members of the group), but I did not feel that I had much to offer as 
James and Nick seemed to be sorting things out on their own.  James' 
resolution of the problem, suggesting additions to the DDL1 dictionary, 
sounds like a good fix which certainly encapsulates the essence of what 
we were thinking when we developed the _atom_site section of the core 
dictionary.  The separate aniso loop was put in because many people 
seemed to like keeping the positional and displacements parameters in 
separate tables, and this was the convention adopted by SHELX and other 
software, presumably in order to keep each row of the table on one 80 
character line.  The _atom_site_aniso_label was added only because STAR 
did not allow _atom_site_label to be repeated in the second loop.  I 
should point out, however, that COMCIFS does not have authority to 
change or approve DDLs, it can only approve CIF dictionaries.  I am not 
sure who is in charge of DDLs, probably Nick and Syd.

As Nick pointed out we developed CIF by the seat of our pants.  The 
first CIF dictionary was conceived as a typeset printed document and it 
was only later that it was realized that it could be typeset by storing 
the dictionary on a computer as a STAR document.  Still later it was 
realized that a STAR dictionary could be used to validate CIFs and even 
later that it was realized that CIFs could have a relational structure.  
Thus DDL was developed on the fly to accommodate CIF dictionaries that 
were already well developed.  During this period Acta Cryst. was tooling 
up to accept structure reports in CIF and decisions had to be made 
quickly at a time when it was impossible to foresee all the implications 
of what we were doing.  There were also compromises that were thought 
necessary to make CIF acceptable to the community, and it was in this 
spirit that Acta Cryst. accepted many CIFs into its archives that were 
not strictly CIF conformant.

Software has taken a long time to catch up with the potential of what 
was designed into CIF and its DDLs.  Browser-editors that validate 
coreCIFs against the dictionary have only appeared in the last couple of 
years, more than a decade after the release of the core dictionary, and 
even these do not validate the relational structure.  By hindsight 
(i.e., with ten years experience as well as the appearance of XML) it is 
clear that we should have done some things differently, and at Florence 
we need to review the whole question of where CIF goes from here.  We 
may decide that we need to adopt starDDL which has been more carefully 
thought out, but there will be a cost.  All the dictionaries will need 
to be revised, the changes will have to be sold to the community and the 
trauma of transition will have to be minimized.  It would, however, give 
us a chance to get it right the second time.

Apparently there has been a systems failure in Chester, which is why 
there is been such a stunning silence from that quarter.

David Brown

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org:McMaster University;Brockhouse Institute for Materials Research
adr:;;King St. W;Hamilton;Ontario;L8S 4M1;Canada
email;internet:idbrown@mcmaster.ca
title:Professor Emeritus
tel;work:+905 525 9140 x 24710
tel;fax:+905 521 2773
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