source: trunk/tutorial2/garnet6.html @ 930

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1<html>
2<head>
3   <title>EXPGUI Tutorial #6</title>
4</head>
5
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15<?
16   include("/var/www/include/navigation.inc");
17   include("/var/www/include/utility.inc");
18?>
19<blockquote>
20
21<CENTER>
22<H1>EXPGUI Tutorial: Garnet (part 6)</H1>
23</CENTER>
24<H1>
256. Additional Least-Squares Refinement
26</H1>
27The initial fit obtained in the previous runs is not particularly good. The
28<A HREF="garnet5.html#diff">"first derivative"</A>
29appearance in the difference plot peaks indicates that
30peaks are not correctly positioned, so refining the lattice parameters and
31zero point correction are the first order of business.
32<BR clear=left><P>
33<img src="fit1.gif">
34<BR clear=left>
35While normally I would recommend refining the lattice first and then
36in a subsequent refinement cycle also refining the zero point correction
37in a later cycle of refinement, with a cubic cell and a small
38zero-point error it is possible to refine both together. Set the refinement
39flag for the lattice (shown above) and the zero point correction (shown below).
40<BR clear=left><P>
41<img src="fit2.gif">
42<BR clear=left><P>
43and then start GENLES, as was done before.
44<BR clear=left><P>
45<img src="fit3.gif">
46<BR clear=left>
47A dramatic improvement is seen both through the agreement factors (above)
48and as seen in the plot (below).
49<BR clear=left><P>
50<a name="ticks"><img src="fit4.gif"></a>
51<BR clear=left><P>
52At this point it is a good idea to run POWPREF and GENLES again,
53since peak positions have moved. This causes the Reduced Chi**2 to drop from
5414.9 to 13.5.
55
56<P><BR>(.EXP Snapshot:
57<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet6.exp">
58garnet6.exp
59</A>)
60
61<BR clear=left><HR>
62Previous step: <A HREF="garnet5.html">
63Plot data using POWPLOT or LIVEPLOT</A>
64</A>
65<BR>
66Next step: <A HREF="garnet7.html">
67Define Constraints</A>
68
69<HR>
70</blockquote>
71<P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR>
72<font size=-1><? lastmod(); ?> </font>
73<BR>$Revision: 930 $ $Date: 2009-12-04 23:14:35 +0000 (Fri, 04 Dec 2009) $
74</body>
75</html>
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