source: trunk/tutorial2/garnet7.html @ 469

Last change on this file since 469 was 192, checked in by toby, 12 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 #7</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 7)</H1>
25</CENTER>
26<H1>
277. Define constraints
28</H1>
29The next step in the refinement is to vary atomic positions, displacement
30(thermal amplitude) parameters and since two sites are
31shared in this material, site occupancies. It is almost never possible to
32refine different displacement parameters or position parameters for
33two atoms that share a site, so one must use constraints to require that
34coordinates and thermal parameters be grouped. With only a single data set
35it is only possible to refine a single occupancy value for a site, so it is
36also necessary to constrain the site occupancies to they add to 1.
37<P>
38For this example, we need to constrain the pairs of atoms that share a site,
39Fe2/Al3 and Al4/Fe5.
40This can be done using the constraints pane. In this case, both atoms are
41on special positions so the coordinates are fixed by symmetry and do
42not need to be constrained. This, however,
43will be ignored to illustrate how coordinates are constrained.
44<BR clear=left>
45<img src="constr1.gif">
46<BR clear=left>
47<img src="constr2.gif" align=left>
48<img src="constr3.gif" align=right>
49Pressing the "New Constraint" button causes the box to the left to appear.
50Select phase 1 at the top, the atoms Fe2 and Al3 by "dragging"
51over them with the mouse (clicking with the mouse while holding the
52control key also works). Select the "XYZU" variable to group x, y, z and Uiso
53for the selected atoms, as shown to the right. Press "Save" and the four
54constraints are generated and appear on the Constraints pane.
55<BR clear=left>
56<BR clear=right>
57<img src="constr4.gif" align=left>
58To constrain the fractional occupancies, again press the "New Constraint"
59button and then press the "New Column" button, to create a second
60constraint box. Select phase 1 for both boxes, Fe2 in one box and Al3
61in the other variable "Frac" in the left-most box. Use
62any values for the multiplier that are equal in magnitude
63but opposite in sign.
64Press "Save" and this constraint is also generated and it appears on the
65Constraints pane, as is shown below.
66<BR clear=left>
67<img src="constr5.gif">
68<BR clear=left>
69The previous two steps are then repeated for Al4 and Fe5, to generate an
70additional five more constraints, as is shown below.
71<BR clear=left>
72<img src="constr6.gif">
73<BR clear=left>
74
75<P><BR>(.EXP Snapshot:
76<A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet7.exp">
77garnet7.exp
78</A>)
79
80<BR clear=left><HR>
81Previous step: <A HREF="garnet6.html">
82Additional Least-Squares Refinement</A>
83<BR>
84Next step: <A HREF="garnet8.html">
85Vary atomic parameters</A>
86</A>
87
88
89<BR clear=left>
90<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>)
91<BR clear=left>
92$Revision: 192 $ $Date: 2009-12-04 23:01:56 +0000 (Fri, 04 Dec 2009) $
93</body>
94</html>
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