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Re: Neutron diffraction experiments



	The _refine_diff_density_ fields are used for diagnostic purposes
to ensure that the refined model gives an adequate fit to the
measurements.  It is not used to record values of the electron density at
specific points in the crystal in a way that might be useful for
theoretical studies on electron distribution.  That being so, the units of
these fields could be different, depending on the nature of the experiment
being performed.  For x-ray diffraction, the units would be e/A^3, for
neutrons they would be fm/A^3.  These fields have no significance apart
from the the experiment they describe and so are meaningless if removed
from their associated R factors and other descriptors of the particular
experiment.

	I would favour just extending the definition by relating the units
to the units used for the scattering factors.  This should be
possible, although I am not quite sure how one does this.  Otherwise we
need to add something to the name, but the name seems wisely to omit any
direct reference to electron density and we should capitalise on this
fortunate circumstance.

	I think we have covered all the other differences between neutron
and x-ray diffraction, but one day we may have to worry about electron
diffraction which now seems to be entering the single crystal structure
determination field.

				David


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



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