source: trunk/doc/expgui6.html

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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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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
49<a href="#AtomConstraints"><B>Atom Constraints</B></A> for normal phases,
50<a href="#MMConstraints"><B>Macromolecular Atom Constraints</B></A>
51  when a macromolecular phase is present, or
52<a href="#ProfileConstraints"><B>Peak Profile Constraints</B></A>.
53GSAS offers many other types of restraints, but these must be
54  accessed in EXPEDT.
55<P>
56Note that in GSAS these constraints are only applied to shifts, but not to the
57actual values. This means that, for example,
58if two atoms
59are constrained so that the shift on their occupancy is the same, but
60the initial occupancies differ, the difference in their occupancies
61  will not change as the values are refined.
62<P>
63Due to the way GSAS implements constraints, a parameter can be included
64in only one constraint. Should one or more parameters be constrained twice,
65the refinement results are unpredictable, so EXPGUI will not allow
66  this (EXPEDT will). One can sometimes bypass
67  this problem by defining redundant parameters. For example to
68  constrain the occupancy of site A to B and site A to C, one can
69  create two atom A sites: A<sub>1</sub> & A<sub>2</sub>. Then one can create two
70  constraints, for site A<sub>1</sub> to B and site A<sub>2</sub> to C. The actual occupancy
71  of site A is then that of A<sub>1</sub> plus A<sub>2</sub>
72
73</DL></DL>
74<a name="AtomConstraints"></a>
75<H4>A.6.1 Atom Constraints</H4>
76<DL><DL>
77The atom constraints editing panel appears below. Note that constraints can
78be listed sorted by constraint number, phase number, lowest atom number or
79variable by clicking on the buttons at the top of the panel.
80<P>
81To delete one or more constraints, click on the appropriate
82  checkbuttons
83  in the <B>Delete</B> column and the press the Delete button, below.
84</DL></DL>
85<img SRC="mc.gif" align=TEXTTOP  alt="EXPGUI Screen snapshot">
86<DL><DL>
87<a name="AtomConstraintsError"></a>
88Should parameters be included in more than one constraint,
89an error message is displayed, as shown below.
90Note that the constraint number is displayed as red on the main
91atomic constraints window (see above), if the
92constraint contains a parameter referenced elsewhere.
93Negative contraint values are highlighted with a beige background
94to make them more visible.
95
96</DL></DL>
97<img SRC="mcerr.gif" align=TEXTTOP  alt="EXPGUI Screen snapshot">
98<DL><DL>
99<a name="EditAtomConstraints"></a>
100<P>
101<img SRC="mcedit.gif" align=LEFT  alt="EXPGUI Screen snapshot">
102<img SRC="mcvar.gif" align=RIGHT alt="EXPGUI Screen snapshot">
103When the edit button for a constraint is pressed, the constraint is
104opened for editing. Controls are available to select the phase, variable,
105multiplier and atom(s). Note that for atom selection, use of the control key
106and the left mouse button allow individual atoms to be selected or deselected.
107Shift and the left mouse button allow ranges of atoms to be selected and the
108right mouse button selects all atoms
109<A href="expgui.html#mouseactions">(see Mouse Actions)</A>.
110<P>
111Note that it is inappropriate
112to constrain different types of parameters together, for example X and Uiso.
113The left-most variable button is the "master" which
114determines what variable are allowed in the
115other variable buttons.
116<P>
117When the "New Constraint" button is pressed, a blank constraint editing
118window is opened. Note that any variable to the right may then be selected,
119including three special codes, Uxx, XYZU and XYZU+-F.
120<UL><LI>
121XYZU creates
122four constraints,
123requiring the shifts on x, y, z and Uiso to be the same for all the selected
124atoms.
125<LI> 
126Uxx does the same for the six anisotropic displacement parameters.
127<LI>
128<a name="XYZUF"></a>XYZU+-F is used for pairs of atoms where
129the shifts on x, y, z and Uiso are required to be equal, but the shifts
130on Frac are opposite.
131</UL>
132These special codes are valuable for atoms that share a site or are
133otherwise grouped together.
134<P>
135<a name="AtomConstraintsMax"></a>
136Note that only 100 parameters may be included in a single atomic constraint,
137with one exception. When shifts for Uiso
138are constrained to the same value for all atoms in a phase,
139this counts as a single parameter. If more than 500
140parameters are entered for a constraint, an error message is shown.
141Up to 2000 total atomic constraints may be entered.
142EXPGUI does not warn if you exceed this limit.
143</DL></DL>
144
145<a name="MMConstraints"></a>
146<H4>A.6.2 Macromolecular Constraints</H4>
147<DL><DL>
148Constraints for a macromolecular phase can be entered on a
149panel similar to the Atomic Constraints, except that the phase number
150does not appear anywhere (since the macromolecular phase must
151be phase #1). The variables that can be constrained for
152a macromolecular phase are the coordinates, x, y, & z as well as
153the occupancy, FRA and the atomic displacement parameter, UIS.
154The special constraint, XYZU, is used to constrain all coordinates
155and the displacement parameter.
156</DL></DL>
157
158<a name="ProfileConstraints"></a>
159<H4>A.6.3 Profile Constraints</H4>
160<DL><DL>
161The profile constraints editing panel appears below. Profile constraints
162are used to relate the shifts applied to profile terms for different
163phase/histogram combinations. Profile terms are identified by number,
164so while EXPEDT allows one to constrain the first profile term for
165a TOF histogram to a the first term in a CW histogram, EXPGUI attempts to
166make this impossible.
167<P>
168To delete one or more constraints, click on the appropriate checkbuttons
169in the <B>Delete</B> column and the press the Delete button, below.
170</DL></DL>
171<img SRC="pc.gif" align=TEXTTOP alt="EXPGUI Screen snapshot">
172<DL><DL>
173
174<a name="ProfileConstraintErr"></a>
175<img SRC="pcerr.gif" align=LEFT alt="EXPGUI Screen snapshot">
176Should parameters be included in more than one constraint,
177an error message is displayed, as shown below.
178Note that the constraint number is displayed as red on the main
179profile constraints window (see above), if the
180constraint contains a parameter referenced elsewhere.
181
182<BR CLEAR=LEFT>
183<P>
184<a name="EditProfileConstraints"></a>
185When the edit button for a constraint is pressed, the constraint is
186opened for editing, as shown below.
187Listboxes contain the available phases and histograms.
188Only the histograms of the same data type (TOF, CW or ED) are included,
189but no check is made to ensure that the
190selected phase/histogram combinations
191all have the same profile type.
192Note that for phase and histogram selection, use of the control key
193and the left mouse button allow individual items to be selected or deselected.
194Shift and the left mouse button allow ranges of items to be selected and the
195right mouse button selects all phases or histograms.
196<A href="expgui.html#mouseactions">(see Mouse Actions)</A>.
197<P>
198<img SRC="pcedit.gif" align=TEXTTOP alt="EXPGUI Screen snapshot">
199<P>
200</DL></DL>
201<a name="NewProfileConstraints"></a>
202<img SRC="pcnew1.gif" align=LEFT alt="EXPGUI Screen snapshot">
203When the "New Constraint" button is pressed, a window is opened to select
204the histogram and profile function type to be constrained. Note that
205only for profile function #4, where the number of terms depends on the
206phase symmetry, does it matter what phase you select. The available profile
207terms are displayed on the bottom of the window and more than
208one can be selected.
209<BR CLEAR=LEFT>
210<P>
211<DL><DL>
212<img SRC="pcnew2.gif" align=LEFT alt="EXPGUI Screen snapshot">
213
214After the "Continue" button is pressed, a constraint
215edit window is opened, as shown to the left.
216Listboxes contain the available phases and histograms and
217only the histograms of the selected data type are included, as before.
218Also, for phase and histogram selection, use of the control or shift key
219and the left mouse button allow individual items to be selected or deselected
220and the right mouse button selects all phases or histograms.
221Note that if more than one profile term was selected on the previous window,
222the same set of constraints are for each term.
223<BR CLEAR=LEFT>
224<P>
225<a name="ProfileConstraintsMax"></a>
226Note that only 27 terms can be included in a single profile constraint.
227Also, only like profile terms can be constrained. While GSAS does allow
228different types of profile terms to be constrained together, this does not
229usually make sense and EXPGUI does not allow it.
230</DL></DL>
231<hr>
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233<TR><TH><A  Href="expgui.html">EXPGUI top</A> 
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237
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