source: trunk/tutorial2/garnet8.html @ 193

Last change on this file since 193 was 193, checked in by toby, 11 years ago

# on 2000/06/01 16:07:21, toby did:
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1<html>
2<head>
3   <META NAME="Author" CONTENT="Brian H. Toby">
4   <title>EXPGUI Tutorial #8</title>
5</head>
6<BODY BGCOLOR="#FFFFFF">
7
8<A HREF=http://www.ncnr.nist.gov>
9<IMG SRC="http://www.ncnr.nist.gov/images/ncnrtrans.gif" 
10alt="Link to NIST Center for Neutron Research home page"
11ALIGN=RIGHT></A>
12<A HREF=http://www.nist.gov>
13<IMG SRC="http://www.ncnr.nist.gov/images/webidblue_2lineright.gif" 
14alt="Link to National Institute of Standards & Technology home page"
15ALIGN=LEFT></A>
16<CENTER>
17<A Href="http://www.ncnr.nist.gov/programs/crystallography/software/tclpkgs.html">
18<IMG SRC="tcltklogo100.gif" 
19alt="Link to Tcl/Tk information">
20</CENTER></A>
21<hr>
22
23<CENTER>
24<H1>EXPGUI Tutorial: Garnet (part 8)</H1>
25</CENTER>
26<H1>
278. Vary atomic parameters
28</H1>
29Now that parameters have been appropriately constrained, they can be varied.
30Below all atoms are selected (using a mouse drag or a right-mouse click)
31and the "U" and "X" flags are set, so that all allowed coordinates and
32all isotropic displacement parameters are set to be varied. Note that
33all but the last atom are located on special positions where the
34coordinates are fixed
35by symmetry.
36Thus, it makes no difference if the "X" flag is set for the
37first five atoms.
38<BR clear=left><P>
39<img src="varatom1.gif">
40<BR clear=left><P>
41The four shared-site atoms are selected and the "F" (fractional occupancy)
42flag is set for those atoms.
43<BR clear=left><P>
44<img src="varatom2.gif">
45<BR clear=left>
46At this point GENLES can be run.
47<BR clear=left><P>
48<img src="varatom3.gif">
49<BR clear=left><P>
50Significant improvement is noted, both by agreement factors (see above)
51and from the plot (below).
52<BR clear=left><P>
53<img src="varatom4.gif">
54<BR clear=left>
55Still more improvement (see below)
56can be obtained by allowing the first four profile
57parameters (U, V, W and asym) and increasing the number of
58background terms. For CW neutron
59data, I recommend background function #1, the Chebychev polynomial function,
60and 6-10 terms, for a fairly flat background such as this.
61
62<BR clear=left><P>
63<img src="varatom5.gif">
64<BR clear=left>
65
66<P><BR>(.EXP Snapshot:
67<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet8.exp">
68garnet8.exp
69</A>)
70
71<BR clear=left><HR>
72Previous step: <A HREF="garnet7.html">
73Define constraints
74</A>
75<BR>
76Next step: <A HREF="garnet9.html">
77Compute Bond Distances and Angles</A>
78
79<BR clear=left>
80<HR><p><a href="http://www.ncnr.nist.gov/staff/toby/">Brian Toby</a> (<a href="mailto:brian.toby@nist.gov">Brian.Toby@NIST.GOV</a>)
81<BR clear=left>
82$Revision: 193 $ $Date: 2009-12-04 23:01:57 +0000 (Fri, 04 Dec 2009) $
83</body>
84</html>
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