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Resolution limits



For the benefit of members of the core DMG I enclose a letter received
from Phillip Fenwick who is concerned about the lack of a theta(max) field
in the refine category.  I have replied to him explaining our philosophy
on this point (that resolution in A is more important than theta max in a
refinement since the wavelengths can vary), but, in the spirit of
providing the members of the DMG with feedback from the community, I post
Phillip's letter.  The increased use of area detectors may cause us to
revisit this topic, though my feeling is that we are on the right track,
since the increasing use of synchrotrons will mean that not all structures
will be determined at the wavelength of MoKa. 


*****************************************************
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
*****************************************************

Dear David;

I have run into a problem that the current CIF dictionaries do not easily
address.  I am collecting data on a CCD and usually collect to some large
value of theta.  However, in some cases it is clear that there is little or
no high angle data and I therefore limit the theta value in the refinement.
This is easily done in SHELXL.  While I want to report the theta angles for
data collection in "diffrn_reflns_theta_max", I also feel I need to report
this theta restriction in ls.  There is currently nothing like
"refine_ls_theta_max".  There is a report of the d values used but I doubt many
would bother to convert these back to  theta values so they could be compared
to the data collected values.  While I realize macromolecular crystallographers
think in terms of d most small molecule guys think in theta.

The end result of not reporting the theta reduction in ls is that it makes the
number of data collected and refined look very different without providing
a clear explanation.  In the old days of diffractometers, the data collection
was simply stopped when a shell indicated there was no data.  However, with the
CCD this is not a practical approach.

This is just something to consider.

Best wishes
Phil  

Phillip Fanwick

Dept. of Chemistry; Purdue University; W. Lafayette, IN 47907-1393
tel: 765-494-4572             fax: 765-494-0239
email: fanwick@xray.chem.purdue.edu




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