Used to be: Coordinates accuracy
Howard Flack
Howard.Flack at cryst.unige.ch
Thu Feb 27 05:00:59 EST 1997
Lynn,
In some of my spare moments (so few), I think about the presentation
you made at the computing school in Western Washington with the
analysis of eigenvalues and vectors of the normal-equations matrix for
macromolecular refinements. One thing that bothers me is that the
eigenvalue spectrum can be changed by making a linear transformation
of the variables from x(i) to s(i)x(i) where s(i) is an arbitrary
scale factor. [You can also instead of 'refining on |F(h)|' refine on
k(h).|F(h)| where k(h) is some arbitrary scale factor for each
reflection. As the weights transform inversely to |F|^2^, this has no
effect on the eigenvalues.] I would not expect the real nature of the
refinement to be altered by such a silly trick but one can nevertheless
easily arrange for all the eigenvalues to have the same value. This
seems to suggest that the eigenvalues are not the invariants that one
needs to examine or may be I am talking utter nonsense?
Best wishes,
Howard
--
Howard Flack http://www.unige.ch/crystal/ahdf/Howard.Flack.html
Laboratoire de Cristallographie Phone:(+41 22) 702 62 49
24 quai Ernest-Ansermet mailto:Howard.Flack at cryst.unige.ch
CH-1211 Geneva 4, Switzerland Fax:(+41 22) 781 21 92
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