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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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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="expgui6A.html">Next page</A>
25</TH><TH><A Href="expgui6.html">Previous page</A>
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27
28<center><h1>
29<HR noshade width="75%" size="2" align="center">
30EXPGUI, part 7
31<HR noshade width="75%" size="2" align="center">
32</h1></center>
33
34<h3>A.7 Restraints panel</h3>
35<DL><DL>
36GSAS offers two ways to limit changes to parameters that are based on
37  expectations
38  created by a crystallographer to create what is felt to be the most
39  reasonable model.
40  Restraints penalize the fit if
41aspects of a model (for example, bond lengths) differ from selected
42  values. They are sometimes called "soft constraints", but they are
43  not true constraints. They "push" the model towards the expectation(s)
44  specified by the scientist, but do not require that expectation be
45  maintained. Note that each expectation is weighted by the uncertainty
46  (sometimes s.u. or E.S.D.) for the expectation as well as the overall Restraint
47  Weighting factor. The smaller the uncertainty and the larger Restraint
48  Weighting factor, the stronger the model is "pushed" towards
49  following the expectation.
50<P> 
51  Restraints effectively add additional
52  "observations" to the data that are
53  fit in GSAS. Of course, these "observations" are
54  conjectures made by the scientist and are not actually real data.
55These are distingished from constraints (sometimes called "hard
56  constraints") which are listed on the
57<A Href="expgui6.html">Constraints panel</A>). Constraints actually
58  reduce the actual number of refined parameters in a model,
59by grouping parameters so that a single shift (possibly scaled) is
60applied to each parameter in the group.
61<P>
62  The Restraints panel at present can be used to set up restraints on
63interatomic distances and atomic composition.  EXPEDT offers many
64  other types of restraints as well.
65</DL></DL>
66
67<a name="DistanceRestraints"></a>
68<H4>Distance Restraints</H4>
69<DL><DL>
70 When the distance restraints tab is first selected, the active
71  distance restraints for the selected phase are shown. The <B>Atom 1</B> and
72  <B>Atom 2</B> buttons can be used to sort this list.
73  <P>
74  The <B>Restraint Weight</B> parameter is very important. As this
75  determines how strongly the distance restraints will be used in the
76  refinement relative to the diffraction data. This value is sometimes
77  set as high as 500,000 in the initial stages of a fit, but should be
78  lowered as much as is possible in the final stages of the fit, as
79  needed to obtain chemically plausible interatomic distances. For
80  some refinements, a value of 0.0 is possible for the Restraint
81  Weight in the final stages. This removes any restraint effect.
82
83  To see how restraints are affecting the refinement, one can look at
84  the ratio of the "Sum(w*d**2)" (shown in the .LST file) for the
85  diffraction histograms vs. the restraint histogram.
86<P>
87  The <B>Active Restraints Editor</B> can be used to select distances
88  to be restrained or edit the restraints. If there are too many
89  distances shown, the options at the top of the window can be used to
90  select a distance range, or specific atoms to include. Likewise,
91  only restrained or unrestrained distances can be selected. The
92  changes are applied when the <B>Filter</B> button is pressed.
93<P>
94  The bottom section of the window shows individual interatomic
95  distances. A <B>Restraint</B> and <B>Tolerance</B> value is needed to restrain
96  that distance.
97</DL></DL>
98<img src="dist1.png" alt="Distance constraint example screen">.
99<DL><DL>
100
101  The quantity minimized is
102<blockquote>
103<blockquote>
104 "Restraint Weight" * SUM{  [ (Distance - Restraint) / Tolerance ]**2 }
105</blockquote>
106</blockquote>
107thus the "Tolerance" provides individual weighting for each
108  restraint.
109<P>
110  To set a large number of restraints at the same time, select the
111  check button next to the distance(s), or use the <B>Check
112  All</B>. Then use the section above to enter a global
113  <B>Restraint</B> and <B>Tolerance</B> value. These values are set
114  when the <B>Set Checked</B> button is used.  The <B>Delete
115  Checked</B> button removes restraints from all selected distances.
116<P>
117The entered restraint information will be used onlt when the <B>Save
118  Changes</B> button is pressed. The <B>Cancel</B> button will reverse
119  any changes made while the window is open. Closing the window has
120  the same effect as "Cancel."
121  <P>
122  Once distances are entered, they are shown on the main panel, as
123  shown below.
124</DL></DL>
125<img src="dist2.png" alt="Distance constraint example screen">.
126<DL><DL>
127  Note that the interatomic distances that are shown are determined by
128  the parameters to program DISAGL. These are controlled with the
129  <B>Edit Search Distance Parameters</B> button.
130</DL></DL>
131<a name="Chemical Restraints"></a>
132<H4>Chemical (or Compositional) Restraints</H4>
133<DL><DL>
134  Chemical constraints allow a model to be penalized if it moves away
135  from expectations defined on the chemical compositions.
136  Up to 9 constraints, of form
137  <center>
138  <I>Target</I> =
139  W<sub>1</sub> * O<sub>1</sub> * M<sub>1</sub> +
140  W<sub>2</sub> * O<sub>2</sub> * M<sub>2</sub> +
141  W<sub>3</sub> * O<sub>3</sub> * M<sub>3</sub> + ...
142  </center>
143can be defined,
144  where O<sub>i</sub> is the occupancy of atom <I>i</I> (often refined),
145  M<sub>i</sub> is the site multiplicity atom <I>i</I> (determined by
146  the site symmetry) and W<sub>i</sub> and <I>Target</I> are set by
147  the scientist who defines the constraint.
148This allows chemical constraints to be used in many different
149  ways. They can be used to "push" a model to have a particular
150  composition on a site, or "push" an overall unit cell compositon or
151  even encourage that the chemistry follow rules such a balanced
152  charges with fixed atomic valency.
153<P>
154  When the panel is first loaded, it will be mostly blank, as shown below:
155</DL></DL>
156<img src="chem0.png" alt="Chemical constraint example screen">.
157<DL><DL>
158The "Add Constraint" button is used to enter one or more constraints,
159  which are then defined by selecting the phase, and atom name for
160  each atom in the constraint, as well as the weighting factor for
161  each atom (W<sub>i</sub>, above) as well as the Target value for the
162  sum (W<sub>1</sub> * O<sub>1</sub> * M<sub>1</sub> +
163  W<sub>2</sub> * O<sub>2</sub> * M<sub>2</sub> +
164  W<sub>3</sub> * O<sub>3</sub> * M<sub>3</sub> + ...) and the
165  uncertainty (ESD) for the sum, as shown below:
166</DL></DL>
167<img src="chem1.png" alt="Chemical constraint example screen">.
168<DL><DL>
169A more complex example, which implements a charge balance constraint,
170  is shown below.
171</DL></DL>
172<img src="chem2.png" alt="Chemical constraint example screen">.
173<DL><DL>
174Constraints are saved as the values are typed in. Also, the sum of the
175  actual
176W<sub>i</sub> * O<sub>i</sub> * M<sub>i</sub> values is shown in the
177  upper right.
178Atom entries where
179  the weight is blank or set to zero are ignored. New rows (up to 297)
180  are added when the existing ones are all used. If an error is
181  encountered in a constraint, a question mark ("?") is displayed for
182  the Actual Sum and the constraint is not saved.
183
184</DL></DL>
185<hr>
186<TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT>
187<TR><TH><A  Href="expgui.html">EXPGUI top</A> 
188</TH><TH><A Href="expgui6A.html">Next page</A>
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191
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