"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 ******************************************************************** 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 ********************************************************************