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EXPGUI Tutorial: Garnet (part 8)

8. Vary atomic parameters

Now that parameters have been appropriately constrained, they can be varied. Below all atoms are selected (using a mouse drag or a right-mouse click) and the "U" and "X" flags are set, so that all allowed coordinates and all isotropic displacement parameters are set to be varied. Note that all but the last atom are located on special positions where the coordinates are fixed by symmetry. Thus, it makes no difference if the "X" flag is set for the first five atoms.

The four shared-site atoms are selected and the "F" (fractional occupancy) flag is set for those atoms.

At this point GENLES can be run.

Significant improvement is noted, both by agreement factors (see above) and from the plot (below).

Still more improvement (see below) can be obtained by allowing the first four profile parameters (U, V, W and asym) and increasing the number of background terms. For CW neutron data, I recommend background function #1, the Chebychev polynomial function, and 6-10 terms, for a fairly flat background such as this.

(.EXP Snapshot: garnet8.exp )

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Next step: Compute Bond Distances and Angles

Brian Toby (Brian.Toby@NIST.GOV)
$Revision: 193 $ $Date: 2009-12-04 23:01:57 +0000 (Fri, 04 Dec 2009) $