Crystallography Online: the website of the International Union of Crystallography

Joe Krahn wrote:

>How many people are working on the new electron-density dictionary?
>
The electron density dictionary is part of the corpus of approved CIF 
dictionaries, but it is designed to report accurate electron density 
measurement on small cell crystals using atom-centered multipole 
functions.  I suspect you are looking for lower resolution elelctron 
density maps of protein molecules and I am not aware of any group 
working on a CIF representation of that.

>
>So, matrix/vector symmetry should be supported. Are there opinions as to
>whether the equation method should or should not also be supported for
>things like electron-density maps?
>
Dictionaries defined using DDLm (previously DDL3) allow matrices and 
vectors to be reported as a single item.  It is the awkwardness of the 
presentation of matrices in DDL1 and DDL2 dictionaries that discouraged 
us from defining a matrix+vector representation of symmetry operations 
in the current suite of dictionaries.  CIF also supports the use of the 
Hall space group symbol which uniquely defines the space group 
operations, hence the space group and its setting.  It is thus more 
informative than the Hermann-Mauguin symbol which does not uniquely 
specify the origin.

David Brown
Chair COMCIFS