source: trunk/tutorial2/RCS/garnet7.html,v @ 932

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25desc
26@Garnet tutorial
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33@
34text
35@<html>
36<head>
37   <title>EXPGUI Tutorial #7</title>
38</head>
39
40<style>
41A:link {text-decoration:none}
42A:vlink {text-decoration:none}
43</style>
44
45<BODY BGCOLOR="#FFFFFF"
46      topmargin="0" leftmargin="0" marginwidth="0" marginheight="0"
47      text="#000000" link="#0033ff" vlink="#0033ff" alink="#0033ff">
48
49<?
50   include("/var/www/include/navigation.inc");
51   include("/var/www/include/utility.inc");
52?>
53<blockquote>
54
55<CENTER>
56<H1>EXPGUI Tutorial: Garnet (part 7)</H1>
57</CENTER>
58<H1>
597. Define constraints
60</H1>
61The next step in the refinement is to vary atomic positions, displacement
62(thermal amplitude) parameters and since two sites are
63shared in this material, site occupancies. It is almost never possible to
64refine different displacement parameters or position parameters for
65two atoms that share a site, so one must use constraints to require that
66coordinates and thermal parameters be grouped. With only a single data set
67it is only possible to refine a single occupancy value for a site, so it is
68also necessary to constrain the site occupancies to they add to 1.
69<P>
70For this example, we need to constrain the pairs of atoms that share a site,
71Fe2/Al3 and Al4/Fe5.
72This can be done using the constraints pane. In this case, both atoms are
73on special positions so the coordinates are fixed by symmetry and do
74not need to be constrained. This, however,
75will be ignored to illustrate how coordinates are constrained.
76<BR clear=left>
77<img src="constr1.gif">
78<BR clear=left>
79<img src="constr2.gif" align=left>
80<img src="constr3.gif" align=right>
81Pressing the "New Constraint" button causes the box to the left to appear.
82Select phase 1 at the top, the atoms Fe2 and Al3 by "dragging"
83over them with the mouse (clicking with the mouse while holding the
84control key also works). Select the "XYZU" variable to group x, y, z and Uiso
85for the selected atoms, as shown to the right. Press "Save" and the four
86constraints are generated and appear on the Constraints pane.
87<BR clear=left>
88<BR clear=right>
89<img src="constr4.gif" align=left>
90To constrain the fractional occupancies, again press the "New Constraint"
91button and then press the "New Column" button, to create a second
92constraint box. Select phase 1 for both boxes, Fe2 in one box and Al3
93in the other variable "Frac" in the left-most box. Use
94any values for the multiplier that are equal in magnitude
95but opposite in sign.
96Press "Save" and this constraint is also generated and it appears on the
97Constraints pane, as is shown below.
98<BR clear=left>
99<img src="constr5.gif">
100<BR clear=left>
101The previous two steps are then repeated for Al4 and Fe5, to generate an
102additional five more constraints, as is shown below.
103<BR clear=left>
104<img src="constr6.gif">
105<BR clear=left>
106
107<P><BR>(.EXP Snapshot:
108<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet7.exp">
109garnet7.exp
110</A>)
111
112<BR clear=left><HR>
113Previous step: <A HREF="garnet6.html">
114Additional Least-Squares Refinement</A>
115<BR>
116Next step: <A HREF="garnet8.html">
117Vary atomic parameters</A>
118</A>
119
120<HR>
121</blockquote>
122<P><font size=-1><A HREF="MAILTO:crystal@@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@@NIST.gov</A></font><BR>
123<font size=-1><? lastmod(); ?> </font>
124<BR>$Revision: 1.4 $ $Date: 2002/04/12 21:21:18 $
125</body>
126</html>
127@
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134text
135@a2 1
136   <META NAME="Author" CONTENT="Brian H. Toby">
137a4 1
138<BODY BGCOLOR="#FFFFFF">
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141<A HREF=http://www.ncnr.nist.gov>
142<IMG SRC="http://www.ncnr.nist.gov/images/ncnrtrans.gif"
143alt="Link to NIST Center for Neutron Research home page"
144ALIGN=RIGHT></A>
145<A HREF=http://www.nist.gov>
146<IMG SRC="http://www.ncnr.nist.gov/images/webidblue_2lineright.gif"
147alt="Link to National Institute of Standards & Technology home page"
148ALIGN=LEFT></A>
149<CENTER>
150<A Href="http://www.ncnr.nist.gov/programs/crystallography/software/tclpkgs.html">
151<IMG SRC="tcltklogo100.gif"
152alt="Link to Tcl/Tk information">
153</CENTER></A>
154<hr>
155d86 5
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157
158<BR clear=left>
159<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>)
160<BR clear=left>
161$Revision: 1.1 $ $Date: 2000/05/30 22:22:38 $
162@
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167@Initial revision
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169text
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172addtional five more constraints, as is shown below.
173d92 1
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175$Revision: 1.1 $ $Date: 2000/05/26 16:08:48 $
176@
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