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| 2 | <head> |
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| 3 | <title>EXPGUI Tutorial #8</title> |
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| 7 | A:link {text-decoration:none} |
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| 11 | <BODY BGCOLOR="#FFFFFF" |
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| 13 | text="#000000" link="#0033ff" vlink="#0033ff" alink="#0033ff"> |
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| 14 | |
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| 15 | <? |
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| 16 | include("/var/www/include/navigation.inc"); |
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| 17 | include("/var/www/include/utility.inc"); |
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| 18 | ?> |
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| 19 | <blockquote> |
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| 20 | |
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| 21 | <CENTER> |
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| 22 | <H1>EXPGUI Tutorial: Garnet (part 8)</H1> |
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| 23 | </CENTER> |
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| 24 | <H1> |
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| 25 | 8. Vary atomic parameters |
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| 26 | </H1> |
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| 27 | Now that parameters have been appropriately constrained, they can be varied. |
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| 28 | Below all atoms are selected (using a mouse drag or a right-mouse click) |
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| 29 | and the "U" and "X" flags are set, so that all allowed coordinates and |
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| 30 | all isotropic displacement parameters are set to be varied. Note that |
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| 31 | all but the last atom are located on special positions where the |
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| 32 | coordinates are fixed |
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| 33 | by symmetry. |
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| 34 | Thus, it makes no difference if the "X" flag is set for the |
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| 35 | first five atoms. |
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| 36 | <BR clear=left><P> |
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| 37 | <img src="varatom1.gif"> |
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| 38 | <BR clear=left><P> |
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| 39 | The four shared-site atoms are selected and the "F" (fractional occupancy) |
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| 40 | flag is set for those atoms. |
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| 41 | <BR clear=left><P> |
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| 42 | <img src="varatom2.gif"> |
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| 43 | <BR clear=left> |
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| 44 | At this point GENLES can be run. |
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| 45 | <BR clear=left><P> |
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| 46 | <img src="varatom3.gif"> |
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| 47 | <BR clear=left><P> |
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| 48 | Significant improvement is noted, both by agreement factors (see above) |
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| 49 | and from the plot (below). |
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| 50 | <BR clear=left><P> |
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| 51 | <img src="varatom4.gif"> |
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| 52 | <BR clear=left> |
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| 53 | Still more improvement (see below) |
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| 54 | can be obtained by allowing the first four profile |
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| 55 | parameters (U, V, W and asym) and increasing the number of |
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| 56 | background terms. For CW neutron |
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| 57 | data, I recommend background function #1, the Chebychev polynomial function, |
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| 58 | and 6-10 terms, for a fairly flat background such as this. |
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| 59 | |
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| 60 | <BR clear=left><P> |
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| 61 | <img src="varatom5.gif"> |
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| 62 | <BR clear=left> |
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| 63 | |
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| 64 | <P><BR>(.EXP Snapshot: |
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| 65 | <A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet8.exp"> |
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| 66 | garnet8.exp |
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| 67 | </A>) |
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| 68 | |
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| 69 | <BR clear=left><HR> |
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| 70 | Previous step: <A HREF="garnet7.html"> |
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| 71 | Define constraints |
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| 72 | </A> |
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| 73 | <BR> |
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| 74 | Next step: <A HREF="garnet9.html"> |
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| 75 | Compute Bond Distances and Angles</A> |
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| 76 | |
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| 77 | <HR> |
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| 78 | </blockquote> |
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| 79 | <P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR> |
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| 80 | <font size=-1><? lastmod(); ?> </font> |
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| 81 | <BR>$Revision: 930 $ $Date: 2009-12-04 23:14:35 +0000 (Fri, 04 Dec 2009) $ |
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| 82 | </body> |
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| 83 | </html> |
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