>U, V, and W tend to be highly correlated The correlation between U,V,W is the result of a resolution curve that doesn't have a well defined minimum. If you differentiate the Caglioti equation for the FWHM: FWHM**2=U.tan**2(theta)+V.tan(theta)+W you obtain the condition for the minimum as 2U.tan(theta) + V = 0 so you can constrain V = -2U.tan(theta) Theta can be assumed to be about half the monochromator take-off angle (2thetaM) for a conventional neutron diffractometer eg for 2thetaM=90 you can just approximate: V=-2U (This was known from the time of Rietveld...)
Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> fax (33) 4.76.20.76.48 (33) 4.76.20.72.13 (.26 Mme Guillermet) http://www.ill.fr/dif/AlanHewat.htm ___________________________________________________________
