Jon Wright said:
> Quick question - does anyone have a trick to stop the Cagliotti formula
> going negative?
This can happen if the resolution is relatively flat, so that there is no
well defined minimum. Then the quadratic Cagliotti formula produces large
correlations between U,V,W. The trick is to constrain the minimum to a
fixed angle; eg in the case of a monochromated white beam, this is the
"parallel" geometry when the scattering angle equals the monochromator
angle. This is an approximation, but better than nothing. You could also
replace the Cagliotti formula by a linear equation if you have access to
the refinement code.
So just differentiate the Cagliotti formula and equate the derivative to
zero to obtain V in terms of U and the fixed angle for minimum half-width.
This works well for classic neutron diffractometers and in general is
probably the best you can do if the refinement programme only allows the
Cagliotti formula.
Alan.
______________________________________________
Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE
<alan.he...@neutronoptics.com> +33.476.98.41.68
http://www.NeutronOptics.com/hewat
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