Crystallography Online: the website of the International Union of Crystallography

David Brown wrote:
> 
> 
> 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.
I had written to Herbert a few weeks back and said "Shouldn't there be a
CIF electron-density map specification?", to which he replied that such
a thing had just recently been discussed. I suspect he was referring to
the current hi-res density specification. In any case, it would be good
to incorporate low-res maps as well.

> 
>>
>> 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.
> 
What about equation forms? It is important to define more than just the
space group, because non-standard settings can be useful in some cases.
It is also useful to include matrix+vector for such things as a
spherical asymmetric unit from a 72-fold symmetric virus map. If nobody
is working on it, maybe I should go ahead and write down a few
alternatives and check opinions of other macro-molecular crystallographers.

Thanks, Joe Krahn