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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>
7<style>
8A:link {text-decoration:none}
9A:vlink {text-decoration:none}
10</style>
11
12<BODY BGCOLOR="#FFFFFF"
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14      text="#000000" link="#0033ff" vlink="#0033ff" alink="#0033ff">
15
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="expgui6R.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 ways to limit changes to parameters that are based rules
37  created by a crystallographer to create what is felt to be the most
38  reasonable model.
39So called "soft constraints", which are better described as "restraints"
40  penalize the fit if
41aspects of a model (for example, bond lengths) differ from selected
42  values. They are now located on the on the <A Href="expgui6R.html">Restraints panel</A>.
43Actual constraints, which are sometimes called "hard constraints", are
44  on panel described here. They reduce the actual number of
45parameters in a model,
46by grouping parameters so that a single shift (possibly scaled) is
47applied to each parameter in the group.
48At this time the only constraints that can be set in EXPGUI are on atomic
49and profile profile (others can be set in EXPEDT).
50  EXPEDT offers many
51  other types of restraints as well.
52<P>
53Note that these constraints are only applied to shifts, but not to the
54actual values. This means that, for example,
55if two atoms
56are constrained so that the shift on their occupancy is the same, but
57the initial occupancies differ, the difference in their occupancies
58  will not change as the values are refined.
59<P>
60Due to the way GSAS implements constraints, a parameter can be included
61in only one constraint. Should one or more parameters be constrained twice,
62the refinement results are unpredictable, so EXPGUI will not allow
63  this (EXPEDT will). One can sometimes bypass
64  this problem by defining redundant parameters. For example to
65  constrain the occupancy of site A to B and site A to C, one can
66  create two atom A sites: A<sub>1</sub> & A<sub>2</sub>. Then one can create two
67  constraints, for site A<sub>1</sub> to B and site A<sub>2</sub> to C. The actual occupancy
68  of site A is then that of A<sub>1</sub> plus A<sub>2</sub>
69
70</DL></DL>
71<a name="AtomConstraints"></a>
72<H4>Atom Constraints</H4>
73<DL><DL>
74The atom constraints editing panel appears below. Note that constraints can
75be listed sorted by constraint number, phase number, lowest atom number or
76variable by clicking on the buttons at the top of the panel.
77<P>
78To delete one or more constraints, click on the appropriate
79  checkbuttons
80  in the <B>Delete</B> column and the press the Delete button, below.
81</DL></DL>
82<img SRC="mc.gif" align=TEXTTOP  alt="EXPGUI Screen snapshot">
83<DL><DL>
84<a name="AtomConstraintsError"></a>
85Should parameters be included in more than one constraint,
86an error message is displayed, as shown below.
87Note that the constraint number is displayed as red on the main
88atomic constraints window (see above), if the
89constraint contains a parameter referenced elsewhere.
90Negative contraint values are highlighted with a beige background
91to make them more visible.
92
93</DL></DL>
94<img SRC="mcerr.gif" align=TEXTTOP  alt="EXPGUI Screen snapshot">
95<DL><DL>
96<a name="EditAtomConstraints"></a>
97<P>
98<img SRC="mcedit.gif" align=LEFT  alt="EXPGUI Screen snapshot">
99<img SRC="mcvar.gif" align=RIGHT alt="EXPGUI Screen snapshot">
100When the edit button for a constraint is pressed, the constraint is
101opened for editing. Controls are available to select the phase, variable,
102multiplier and atom(s). Note that for atom selection, use of the control key
103and the left mouse button allow individual atoms to be selected or deselected.
104Shift and the left mouse button allow ranges of atoms to be selected and the
105right mouse button selects all atoms
106<A href="expgui.html#mouseactions">(see Mouse Actions)</A>.
107<P>
108Note that it is inappropriate
109to constrain different types of parameters together, for example X and Uiso.
110The left-most variable button is the "master" which
111determines what variable are allowed in the
112other variable buttons.
113<P>
114When the "New Constraint" button is pressed, a blank constraint editing
115window is opened. Note that any variable to the right may then be selected,
116including three special codes, Uxx, XYZU and XYZU+-F.
117<UL><LI>
118XYZU creates
119four constraints,
120requiring the shifts on x, y, z and Uiso to be the same for all the selected
121atoms.
122<LI> 
123Uxx does the same for the six anisotropic displacement parameters.
124<LI>
125<a name="XYZUF"></a>XYZU+-F is used for pairs of atoms where
126the shifts on x, y, z and Uiso are required to be equal, but the shifts
127on Frac are opposite.
128</UL>
129These special codes are valuable for atoms that share a site or are
130otherwise grouped together.
131<P>
132<a name="AtomConstraintsMax"></a>
133Note that only 100 parameters may be included in a single atomic constraint,
134with one exception. When shifts for Uiso
135are constrained to the same value for all atoms in a phase,
136this counts as a single parameter. If more than 500
137parameters are entered for a constraint, an error message is shown.
138Up to 2000 total atomic constraints may be entered.
139EXPGUI does not warn if you exceed this limit.
140</DL></DL>
141
142<a name="MMConstraints"></a>
143<H4>Macromolecular Constraints</H4>
144<DL><DL>
145Constraints for a macromolecular phase can be entered on a
146panel similar to the Atomic Constraints, except that the phase number
147does not appear anywhere (since the macromolecular phase must
148be phase #1). The variables that can be constrained for
149a macromolecular phase are the coordinates, x, y, & z as well as
150the occupancy, FRA and the atomic displacement parameter, UIS.
151The special constraint, XYZU, is used to constrain all coordinates
152and the displacement parameter.
153</DL></DL>
154
155<a name="ProfileConstraints"></a>
156<H4>Profile Constraints</H4>
157<DL><DL>
158The profile constraints editing panel appears below. Profile constraints
159are used to relate the shifts applied to profile terms for different
160phase/histogram combinations. Profile terms are identified by number,
161so while EXPEDT allows one to constrain the first profile term for
162a TOF histogram to a the first term in a CW histogram, EXPGUI attempts to
163make this impossible.
164<P>
165To delete one or more constraints, click on the appropriate checkbuttons
166in the <B>Delete</B> column and the press the Delete button, below.
167</DL></DL>
168<img SRC="pc.gif" align=TEXTTOP alt="EXPGUI Screen snapshot">
169<DL><DL>
170
171<a name="ProfileConstraintErr"></a>
172<img SRC="pcerr.gif" align=LEFT alt="EXPGUI Screen snapshot">
173Should parameters be included in more than one constraint,
174an error message is displayed, as shown below.
175Note that the constraint number is displayed as red on the main
176profile constraints window (see above), if the
177constraint contains a parameter referenced elsewhere.
178
179<BR CLEAR=LEFT>
180<P>
181<a name="EditProfileConstraints"></a>
182When the edit button for a constraint is pressed, the constraint is
183opened for editing, as shown below.
184Listboxes contain the available phases and histograms.
185Only the histograms of the same data type (TOF, CW or ED) are included,
186but no check is made to ensure that the
187selected phase/histogram combinations
188all have the same profile type.
189Note that for phase and histogram selection, use of the control key
190and the left mouse button allow individual items to be selected or deselected.
191Shift and the left mouse button allow ranges of items to be selected and the
192right mouse button selects all phases or histograms.
193<A href="expgui.html#mouseactions">(see Mouse Actions)</A>.
194<P>
195<img SRC="pcedit.gif" align=TEXTTOP alt="EXPGUI Screen snapshot">
196<P>
197</DL></DL>
198<a name="NewProfileConstraints"></a>
199<img SRC="pcnew1.gif" align=LEFT alt="EXPGUI Screen snapshot">
200When the "New Constraint" button is pressed, a window is opened to select
201the histogram and profile function type to be constrained. Note that
202only for profile function #4, where the number of terms depends on the
203phase symmetry, does it matter what phase you select. The available profile
204terms are displayed on the bottom of the window and more than
205one can be selected.
206<BR CLEAR=LEFT>
207<P>
208<DL><DL>
209<img SRC="pcnew2.gif" align=LEFT alt="EXPGUI Screen snapshot">
210
211After the "Continue" button is pressed, a constraint
212edit window is opened, as shown to the left.
213Listboxes contain the available phases and histograms and
214only the histograms of the selected data type are included, as before.
215Also, for phase and histogram selection, use of the control or shift key
216and the left mouse button allow individual items to be selected or deselected
217and the right mouse button selects all phases or histograms.
218Note that if more than one profile term was selected on the previous window,
219the same set of constraints are for each term.
220<BR CLEAR=LEFT>
221<P>
222<a name="ProfileConstraintsMax"></a>
223Note that only 27 terms can be included in a single profile constraint.
224Also, only like profile terms can be constrained. While GSAS does allow
225different types of profile terms to be constrained together, this does not
226usually make sense and EXPGUI does not allow it.
227</DL></DL>
228<hr>
229<TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT>
230<TR><TH><A  Href="expgui.html">EXPGUI top</A> 
231</TH><TH><A Href="expgui6R.html">Next page</A>
232</TH><TH><A Href="expgui5.html">Previous page</A>
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234
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