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Re: Absolute structure

  • To: Brian McMahon <bm@iucr.org>
  • Subject: Re: Absolute structure
  • From: "I. David Brown" <idbrown@mcmail.CIS.McMaster.CA>
  • Date: Fri, 12 Dec 1997 13:58:02 -0500 (EST)
	Howard and I seem to have quite different ideas about what is
expected in the _reflns_number_* fields.  I would have thought that
*_total would be a number in the thousands, counting every reflection that
was fed separately into the least squares program.  That is the way I read
the definition.  On the other hand, I find myself somewhat perplexed about
what is meant to go into *_Friedel.  I would assume that it would be the
number of reflections that would be left out of the refinement if the
centre of symmetry was assumed, i.e. it would be roughly a half of
*_total.  If this is the case, I do not see the point of this item.  If
Howard is correct, then the number must always be either 1 or 2 and this
is probably not very interesting either.  There may be some virtue in
listing the number of sets of equivalent reflections that are measured, or
that are included in the refinement (the latter always being less than or
equal to the first) to indicate the extent of the redundancy in the use of
symmetry equivalent reflections, but in practice the different sets are
not likely to contain the same number of reflections.  Do we say that 2
sets were included in the refinement when one set contains a single
reflection?  I would suggest that questions relating to the definition of
crystal class and Laue symmetry be referred to symCIF and that whoever
proposed *_Friedel explain what they had in mind and why this is a useful
item to include.

	I like Howard's definitions of _chemical_absolute_configuration
and _chemical_optical_rotation, and we should also consider renaming the
items _refine_ls_abs_structure_* to _refine_ls_absolute_configuration_*
with Howard's enhanced definition. I am becoming more and more convinced
that the names we use should be as precise as we can make them - anything
less is only likely to cause confusion, either because the item has not
been properly thought out, or someone will want to define a different item
that logically should have the name we have already used (incorrectly) for
something else. 

			David


*****************************************************
Dr.I.David Brown
Brockhouse Institute for Materials Research, 
McMaster University, Hamilton, Ontario, Canada
Tel: 1-(905)-525-9140 ext 24710
Fax: 1-(905)-521-2773
*****************************************************

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