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3   <title>EXPGUI Tutorial #7</title>
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22<H1>EXPGUI Tutorial: Garnet (part 7)</H1>
257. Define constraints
27The next step in the refinement is to vary atomic positions, displacement
28(thermal amplitude) parameters and since two sites are
29shared in this material, site occupancies. It is almost never possible to
30refine different displacement parameters or position parameters for
31two atoms that share a site, so one must use constraints to require that
32coordinates and thermal parameters be grouped. With only a single data set
33it is only possible to refine a single occupancy value for a site, so it is
34also necessary to constrain the site occupancies to they add to 1.
36For this example, we need to constrain the pairs of atoms that share a site,
37Fe2/Al3 and Al4/Fe5.
38This can be done using the constraints pane. In this case, both atoms are
39on special positions so the coordinates are fixed by symmetry and do
40not need to be constrained. This, however,
41will be ignored to illustrate how coordinates are constrained.
42<BR clear=left>
43<img src="constr1.gif">
44<BR clear=left>
45<img src="constr2.gif" align=left>
46<img src="constr3.gif" align=right>
47Pressing the "New Constraint" button causes the box to the left to appear.
48Select phase 1 at the top, the atoms Fe2 and Al3 by "dragging"
49over them with the mouse (clicking with the mouse while holding the
50control key also works). Select the "XYZU" variable to group x, y, z and Uiso
51for the selected atoms, as shown to the right. Press "Save" and the four
52constraints are generated and appear on the Constraints pane.
53<BR clear=left>
54<BR clear=right>
55<img src="constr4.gif" align=left>
56To constrain the fractional occupancies, again press the "New Constraint"
57button and then press the "New Column" button, to create a second
58constraint box. Select phase 1 for both boxes, Fe2 in one box and Al3
59in the other variable "Frac" in the left-most box. Use
60any values for the multiplier that are equal in magnitude
61but opposite in sign.
62Press "Save" and this constraint is also generated and it appears on the
63Constraints pane, as is shown below.
64<BR clear=left>
65<img src="constr5.gif">
66<BR clear=left>
67The previous two steps are then repeated for Al4 and Fe5, to generate an
[192]68additional five more constraints, as is shown below.
[174]69<BR clear=left>
70<img src="constr6.gif">
71<BR clear=left>
73<P><BR>(.EXP Snapshot:
74<A HREF="">
78<BR clear=left><HR>
79Previous step: <A HREF="garnet6.html">
80Additional Least-Squares Refinement</A>
82Next step: <A HREF="garnet8.html">
83Vary atomic parameters</A>
88<P><font size=-1><A HREF=" page <?=$PHP_SELF?>">Comments, corrections or questions:</A></font><BR>
89<font size=-1><? lastmod(); ?> </font>
90<BR>$Revision: 599 $ $Date: 2009-12-04 23:08:52 +0000 (Fri, 04 Dec 2009) $
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