| 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 Tutorial #8</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> |
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| 24 | <H1>EXPGUI Tutorial: Garnet (part 8)</H1> |
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| 25 | </CENTER> |
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| 26 | <H1> |
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| 27 | 8. Vary atomic parameters |
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| 28 | </H1> |
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| 29 | Now that parameters have been appropriately constrained, they can be varied. |
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| 30 | Below all atoms are selected (using a mouse drag or a right-mouse click) |
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| 31 | and the "U" and "X" flags are set, so that all allowed coordinates and |
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| 32 | all isotropic displacement parameters are set to be varied. Note that |
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| 33 | all but the last atom are located on special positions where the |
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| 34 | coordinates are fixed |
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| 35 | by symmetry. |
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| 36 | Thus, it makes no difference if the "X" flag is set for the |
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| 37 | first five atoms. |
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| 38 | <BR clear=left><P> |
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| 39 | <img src="varatom1.gif"> |
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| 40 | <BR clear=left><P> |
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| 41 | The four shared-site atoms are selected and the "F" (fractional occupancy) |
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| 42 | flag is set for those atoms. |
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| 43 | <BR clear=left><P> |
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| 44 | <img src="varatom2.gif"> |
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| 45 | <BR clear=left> |
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| 46 | At this point GENLES can be run. |
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| 47 | <BR clear=left><P> |
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| 48 | <img src="varatom3.gif"> |
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| 49 | <BR clear=left><P> |
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| 50 | Significant improvement is noted, both by agreement factors (see above) |
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| 51 | and from the plot (below). |
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| 52 | <BR clear=left><P> |
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| 53 | <img src="varatom4.gif"> |
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| 54 | <BR clear=left> |
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| 55 | Still more improvement (see below) |
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| 56 | can be obtained by allowing the first four profile |
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| 57 | parameters (U, V, W and asym) and increasing the number of |
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| 58 | background terms. For CW neutron |
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| 59 | data, I recommend background function #1, the Chebychev polynomial function, |
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| 60 | and 6-10 terms, for a fairly flat background such as this. |
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| 61 | |
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| 62 | <BR clear=left><P> |
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| 63 | <img src="varatom5.gif"> |
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| 64 | <BR clear=left> |
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| 65 | |
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| 66 | <P><BR>(.EXP Snapshot: |
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| 67 | <A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet8.exp"> |
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| 68 | garnet8.exp |
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| 69 | </A>) |
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| 70 | |
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| 71 | <BR clear=left><HR> |
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| 72 | Previous step: <A HREF="garnet7.html"> |
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| 73 | Define constraints |
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| 74 | </A> |
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| 75 | <BR> |
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| 76 | Next step: <A HREF="garnet9.html"> |
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| 77 | Compute Bond Distances and Angles</A> |
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| 78 | |
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| 79 | <BR clear=left> |
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| 80 | <HR><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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| 81 | <BR clear=left> |
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| 82 | $Revision: 193 $ $Date: 2009-12-04 23:01:57 +0000 (Fri, 04 Dec 2009) $ |
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| 83 | </body> |
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| 84 | </html> |
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