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1<html>
2<head>
3   <title>EXPGUI</title>
4   <meta name="keywords" content="crystallography, Rietveld, diffraction,
5   GSAS, EXPGUI">
6</HEAD>
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16<?
17   include("/var/www/include/navigation.inc");
18   include("/var/www/include/utility.inc");
19?>
20<blockquote><font face="arial, helvetica, sans-serif">
21
22<TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT>
23<TR><TH><A  Href="expgui.html">EXPGUI top</A> 
24</TH><TH><A Href="expgui7.html">Next page</A>
25</TH><TH><A Href="expgui5.html">Previous page</A>
26</TH></TR></TABLE><BR CLEAR=ALL>
27
28<center><h1>
29<HR noshade width="75%" size="2" align="center">
30EXPGUI, part 6
31<HR noshade width="75%" size="2" align="center">
32</h1></center>
33
34<h3>A.6 Constraints panel</h3>
35<DL><DL>
36GSAS offers two types of restraints that can be included in a model.
37So called "soft constraints" or "restraints" penalize the fit if
38aspects of a model (for example, bond lengths) differ from selected values.
39So called "hard constraints" reduce the actual number of
40parameters in a model,
41by grouping parameters so that a single shift (possibly scaled) is
42applied to each parameter in the group.
43The constraints panel is used to set these "hard constraints."
44At this time the only constraints that can be set in EXPGUI are on atomic
45and profile profile (others can be set in EXPEDT).
46<P>
47Note that these constraints are only applied to shifts, but not to the
48actual values. This means that, for example,
49if two atoms
50are constrained so that the shift on their occupancy is the same, but
51the initial occupancies differ, the in occupancy difference will not change
52as the values refine.
53<P>
54Due to the way GSAS implements constraints, a parameter can be included
55in only one constraint. Should one or more parameters be constrained twice,
56the refinement results are unpredictable.
57
58</DL></DL>
59<a name="AtomConstraints"></a>
60<H4>Atom Constraints</H4>
61<DL><DL>
62The atom constraints editing panel appears below. Note that constraints can
63be listed sorted by constraint number, phase number, lowest atom number or
64variable by clicking on the buttons at the top of the panel.
65<P>
66To delete one or more constraints, click on the appropriate checkbuttons in the <B>Delete</B> column and the press the Delete button, below.
67</DL></DL>
68<img SRC="mc.gif" align=TEXTTOP  alt="EXPGUI Screen snapshot">
69<DL><DL>
70<a name="AtomConstraintsError"></a>
71Should parameters be included in more than one constraint,
72an error message is displayed, as shown below.
73Note that the constraint number is displayed as red on the main
74atomic constraints window (see above), if the
75constraint contains a parameter referenced elsewhere.
76Negative contraint values are highlighted with a beige background
77to make them more visible.
78
79</DL></DL>
80<img SRC="mcerr.gif" align=TEXTTOP  alt="EXPGUI Screen snapshot">
81<DL><DL>
82<a name="EditAtomConstraints"></a>
83<P>
84<img SRC="mcedit.gif" align=LEFT  alt="EXPGUI Screen snapshot">
85<img SRC="mcvar.gif" align=RIGHT alt="EXPGUI Screen snapshot">
86When the edit button for a constraint is pressed, the constraint is
87opened for editing. Controls are available to select the phase, variable,
88multiplier and atom(s). Note that for atom selection, use of the control key
89and the left mouse button allow individual atoms to be selected or deselected.
90Shift and the left mouse button allow ranges of atoms to be selected and the
91right mouse button selects all atoms
92<A href="expgui.html#mouseactions">(see Mouse Actions)</A>.
93<P>
94Note that it is inappropriate
95to constrain different types of parameters together, for example X and Uiso.
96The left-most variable button is the "master" which
97determines what variable are allowed in the
98other variable buttons.
99<P>
100When the "New Constraint" button is pressed, a blank constraint editing
101window is opened. Note that any variable to the right may then be selected,
102including three special codes, Uxx, XYZU and XYZU+-F.
103<UL><LI>
104XYZU creates
105four constraints,
106requiring the shifts on x, y, z and Uiso to be the same for all the selected
107atoms.
108<LI> 
109Uxx does the same for the six anisotropic displacement parameters.
110<LI>
111<a name="XYZUF"></a>XYZU+-F is used for pairs of atoms where
112the shifts on x, y, z and Uiso are required to be equal, but the shifts
113on Frac are opposite.
114</UL>
115These special codes are valuable for atoms that share a site or are
116otherwise grouped together.
117<P>
118<a name="AtomConstraintsMax"></a>
119Note that only 100 parameters may be included in a single atomic constraint,
120with one exception. When shifts for Uiso
121are constrained to the same value for all atoms in a phase,
122this counts as a single parameter. If more than 500
123parameters are entered for a constraint, an error message is shown.
124Up to 2000 total atomic constraints may be entered.
125EXPGUI does not warn if you exceed this limit.
126</DL></DL>
127
128<a name="MMConstraints"></a>
129<H4>Macromolecular Constraints</H4>
130<DL><DL>
131Constraints for a macromolecular phase can be entered on a
132panel similar to the Atomic Constraints, except that the phase number
133does not appear anywhere (since the macromolecular phase must
134be phase #1). The variables that can be constrained for
135a macromolecular phase are the coordinates, x, y, & z as well as
136the occupancy, FRA and the atomic displacement parameter, UIS.
137The special constraint, XYZU, is used to constrain all coordinates
138and the displacement parameter.
139</DL></DL>
140
141<a name="ProfileConstraints"></a>
142<H4>Profile Constraints</H4>
143<DL><DL>
144The profile constraints editing panel appears below. Profile constraints
145are used to relate the shifts applied to profile terms for different
146phase/histogram combinations. Profile terms are identified by number,
147so while EXPEDT allows one to constrain the first profile term for
148a TOF histogram to a the first term in a CW histogram, EXPGUI attempts to
149make this impossible.
150<P>
151To delete one or more constraints, click on the appropriate checkbuttons
152in the <B>Delete</B> column and the press the Delete button, below.
153</DL></DL>
154<img SRC="pc.gif" align=TEXTTOP alt="EXPGUI Screen snapshot">
155<DL><DL>
156
157<a name="ProfileConstraintErr"></a>
158<img SRC="pcerr.gif" align=LEFT alt="EXPGUI Screen snapshot">
159Should parameters be included in more than one constraint,
160an error message is displayed, as shown below.
161Note that the constraint number is displayed as red on the main
162profile constraints window (see above), if the
163constraint contains a parameter referenced elsewhere.
164
165<BR CLEAR=LEFT>
166<P>
167<a name="EditProfileConstraints"></a>
168When the edit button for a constraint is pressed, the constraint is
169opened for editing, as shown below.
170Listboxes contain the available phases and histograms.
171Only the histograms of the same data type (TOF, CW or ED) are included,
172but no check is made to ensure that the
173selected phase/histogram combinations
174all have the same profile type.
175Note that for phase and histogram selection, use of the control key
176and the left mouse button allow individual items to be selected or deselected.
177Shift and the left mouse button allow ranges of items to be selected and the
178right mouse button selects all phases or histograms.
179<A href="expgui.html#mouseactions">(see Mouse Actions)</A>.
180<P>
181<img SRC="pcedit.gif" align=TEXTTOP alt="EXPGUI Screen snapshot">
182<P>
183</DL></DL>
184<a name="NewProfileConstraints"></a>
185<img SRC="pcnew1.gif" align=LEFT alt="EXPGUI Screen snapshot">
186When the "New Constraint" button is pressed, a window is opened to select
187the histogram and profile function type to be constrained. Note that
188only for profile function #4, where the number of terms depends on the
189phase symmetry, does it matter what phase you select. The available profile
190terms are displayed on the bottom of the window and more than
191one can be selected.
192<BR CLEAR=LEFT>
193<P>
194<DL><DL>
195<img SRC="pcnew2.gif" align=LEFT alt="EXPGUI Screen snapshot">
196
197After the "Continue" button is pressed, a constraint
198edit window is opened, as shown to the left.
199Listboxes contain the available phases and histograms and
200only the histograms of the selected data type are included, as before.
201Also, for phase and histogram selection, use of the control or shift key
202and the left mouse button allow individual items to be selected or deselected
203and the right mouse button selects all phases or histograms.
204Note that if more than one profile term was selected on the previous window,
205the same set of constraints are for each term.
206<BR CLEAR=LEFT>
207<P>
208<a name="ProfileConstraintsMax"></a>
209Note that only 27 terms can be included in a single profile constraint.
210Also, only like profile terms can be constrained. While GSAS does allow
211different types of profile terms to be constrained together, this does not
212usually make sense and EXPGUI does not allow it.
213</DL></DL>
214<hr>
215<TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT>
216<TR><TH><A  Href="expgui.html">EXPGUI top</A> 
217</TH><TH><A Href="expgui7.html">Next page</A>
218</TH><TH><A Href="expgui5.html">Previous page</A>
219</TH></TR></TABLE>
220
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223<br>$Revision: 930 $ $Date: 2009-12-04 23:14:35 +0000 (Fri, 04 Dec 2009) $
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