| 1 | <html> |
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| 2 | <head> |
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| 3 | <META NAME="Author" CONTENT="Brian H. Toby"> |
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| 4 | <title>EXPGUI</title> |
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| 5 | </head> |
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| 6 | <BODY BGCOLOR="#FFFFFF"> |
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| 7 | |
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| 8 | <A HREF=http://www.ncnr.nist.gov> |
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| 16 | <CENTER> |
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| 17 | <A Href="http://www.ncnr.nist.gov/programs/crystallography/software/tclpkgs.html"> |
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| 18 | <IMG SRC="tcltklogo100.gif" |
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| 20 | </CENTER></A> |
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| 21 | <hr> |
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| 22 | |
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| 23 | <center><h1> |
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| 24 | EXPGUI |
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| 25 | </h1></center> |
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| 26 | <A Href="expgui.html">EXPGUI top</A> |
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| 27 | <h3>A.6 Constraints pane</h3> |
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| 28 | <DL><DL> |
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| 29 | GSAS offers two types of restraints that can be included in a model. |
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| 30 | So called "soft constraints" or "restraints" penalize the fit if |
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| 31 | aspects of a model (for example, bond lengths) differ from selected values. |
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| 32 | So called "hard constraints" reduce the actual number of |
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| 33 | parameters in a model, |
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| 34 | by grouping parameters so that a single shift (possibly scaled) is |
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| 35 | applied to each parameter in the group. |
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| 36 | The constraints pane is used to set these "hard constraints." |
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| 37 | At this time the only constraints that can be set in EXPGUI are on atomic |
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| 38 | parameters (others can be set in EXPEDT). |
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| 39 | <P> |
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| 40 | Note that these constraints are only applied to shifts but not to the |
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| 41 | actual values. This means that, for example, |
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| 42 | if two atoms |
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| 43 | are constrained so that the shift on their occupancy is the same, but |
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| 44 | the initial occupancies differ, the in occupancy difference will not change |
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| 45 | as the values refine. |
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| 46 | <P> |
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| 47 | <H4>Atom Constraints</H4> |
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| 48 | The atom constraints editing pane appears below. Note that constraints can |
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| 49 | be listed sorted by constraint number, phase number, lowest atom number or |
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| 50 | variable by clicking on the buttons at the top of the pane. |
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| 51 | <P> |
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| 52 | To delete one or more constraints, click on the appropriate checkbuttons in the <B>Delete</B> column and the press the Delete button, below. |
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| 53 | </DL></DL> |
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| 54 | <img SRC="mc.gif" align=TEXTTOP> |
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| 55 | <DL><DL> |
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| 56 | |
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| 57 | Due to the way GSAS implements these constraints, a parameter can be included |
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| 58 | in only one constraint. Should one or more parameters be constrained twice, |
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| 59 | an error message is shown as shown below. |
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| 60 | Note that the constraint number is displayed as red, for constraints that |
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| 61 | have conflicts in the constraints editing pane (see above.) |
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| 62 | </DL></DL> |
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| 63 | <img SRC="mcerr.gif" align=TEXTTOP> |
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| 64 | <DL><DL> |
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| 65 | <P> |
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| 66 | <img SRC="mcedit.gif" align=LEFT> |
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| 67 | <img SRC="mcvar.gif" align=RIGHT> |
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| 68 | When the edit button for a constraint is pressed, the constraint is |
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| 69 | opened for editing. Controls are available to select the phase, variable, |
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| 70 | multiplier and atom(s). Note that for atom selection, use of the control key |
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| 71 | and the left mouse button allow individual atoms to be selected or deselected. |
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| 72 | Shift and the left mouse button allow ranges of atoms to be selected and the |
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| 73 | right mouse button selects all atoms |
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| 74 | <A href="expgui.html#mouseactions">(see Mouse Actions)</A>. |
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| 75 | <P> |
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| 76 | Note that it is inappropriate |
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| 77 | to constrain different types of parameters together, for example X and Uiso. |
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| 78 | The left-most variable button is the "master" which |
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| 79 | determines what variable are allowed in the |
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| 80 | other variable buttons. |
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| 81 | <P> |
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| 82 | When the "New Constraint" button is pressed, a blank constraint editing |
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| 83 | window is opened. Note that any variable to the right may then be selected, |
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| 84 | including two special codes, Uxx and XYZU. XYZU creates four constraints, |
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| 85 | requiring the shifts on x, y, z and Uiso to be the same for all the selected |
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| 86 | atoms. Uxx does the same for the six anisotropic displacement parameters. |
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| 87 | These special codes are valuable for atoms that share a site. |
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| 88 | <P> |
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| 89 | Note that only 100 parameters may be included in a constraint, |
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| 90 | with one exception. When shifts for Uiso for all atoms in a phase |
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| 91 | are constrained, this counts as a single parameter. If more than 100 |
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| 92 | parameters are entered for a constraint, an error message is shown. |
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| 93 | Up to 2000 atomic constraints may be entered. |
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| 94 | EXPGUI does not warn if you exceed this limit. |
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| 95 | </DL></DL> |
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| 96 | |
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| 97 | <hr> |
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| 98 | <A Href="expgui.html">EXPGUI top</A> |
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| 99 | |
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| 100 | <p><a href="http://www.ncnr.nist.gov/staff/toby/">Brian Toby</a> (<a href="mailto:brian.toby@nist.gov">Brian.Toby@NIST.GOV</a>) |
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| 101 | <br> |
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| 102 | $Revision: 223 $ $Date: 2009-12-04 23:02:27 +0000 (Fri, 04 Dec 2009) $ |
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| 103 | </body> |
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| 104 | </html> |
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