source: trunk/tutorial2/garnet8.html @ 930

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1<html>
2<head>
3   <title>EXPGUI Tutorial #8</title>
4</head>
5
6<style>
7A:link {text-decoration:none}
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9</style>
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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 8)</H1>
23</CENTER>
24<H1>
258. Vary atomic parameters
26</H1>
27Now that parameters have been appropriately constrained, they can be varied.
28Below all atoms are selected (using a mouse drag or a right-mouse click)
29and the "U" and "X" flags are set, so that all allowed coordinates and
30all isotropic displacement parameters are set to be varied. Note that
31all but the last atom are located on special positions where the
32coordinates are fixed
33by symmetry.
34Thus, it makes no difference if the "X" flag is set for the
35first five atoms.
36<BR clear=left><P>
37<img src="varatom1.gif">
38<BR clear=left><P>
39The four shared-site atoms are selected and the "F" (fractional occupancy)
40flag is set for those atoms.
41<BR clear=left><P>
42<img src="varatom2.gif">
43<BR clear=left>
44At this point GENLES can be run.
45<BR clear=left><P>
46<img src="varatom3.gif">
47<BR clear=left><P>
48Significant improvement is noted, both by agreement factors (see above)
49and from the plot (below).
50<BR clear=left><P>
51<img src="varatom4.gif">
52<BR clear=left>
53Still more improvement (see below)
54can be obtained by allowing the first four profile
55parameters (U, V, W and asym) and increasing the number of
56background terms. For CW neutron
57data, I recommend background function #1, the Chebychev polynomial function,
58and 6-10 terms, for a fairly flat background such as this.
59
60<BR clear=left><P>
61<img src="varatom5.gif">
62<BR clear=left>
63
64<P><BR>(.EXP Snapshot:
65<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet8.exp">
66garnet8.exp
67</A>)
68
69<BR clear=left><HR>
70Previous step: <A HREF="garnet7.html">
71Define constraints
72</A>
73<BR>
74Next step: <A HREF="garnet9.html">
75Compute Bond Distances and Angles</A>
76
77<HR>
78</blockquote>
79<P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR>
80<font size=-1><? lastmod(); ?> </font>
81<BR>$Revision: 930 $ $Date: 2009-12-04 23:14:35 +0000 (Fri, 04 Dec 2009) $
82</body>
83</html>
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