"Maxim V. Lobanov" wrote:
> U, V, and W tend to be highly correlated, with a result that various
> combinations of quite different values can lead to essentially the
> same variance, sigma^2. These three parameters, therefore, do not
> converge in a stable manner when refined simultaneously (Prince,
> 1993).
As Ted Prince also pointed out, if the Cagliotti equation is given an
origin shift, e.g.
U tan2(theta-theta0) + V tan(theta-theta0) + W
where theta0 is fixed at 1/2 the monochromator off-angle, the
correlation in these parameters is greatly reduced.
Brian
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Brian H. Toby, Ph.D. Leader, Crystallography Team
[EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562
voice: 301-975-4297 National Institute of Standards & Technology
FAX: 301-921-9847 Gaithersburg, MD 20899-8562
http://www.ncnr.nist.gov/xtal
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