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| 3 | <title>EXPGUI Tutorial #6</title> |
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| 17 | include("/var/www/include/utility.inc"); |
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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 6)</H1> |
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| 23 | </CENTER> |
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| 24 | <H1> |
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| 25 | 6. Additional Least-Squares Refinement |
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| 26 | </H1> |
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| 27 | The initial fit obtained in the previous runs is not particularly good. The |
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| 28 | <A HREF="garnet5.html#diff">"first derivative"</A> |
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| 29 | appearance in the difference plot peaks indicates that |
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| 30 | peaks are not correctly positioned, so refining the lattice parameters and |
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| 31 | zero point correction are the first order of business. |
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| 32 | <BR clear=left><P> |
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| 33 | <img src="fit1.gif"> |
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| 34 | <BR clear=left> |
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| 35 | While normally I would recommend refining the lattice first and then |
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| 36 | in a subsequent refinement cycle also refining the zero point correction |
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| 37 | in a later cycle of refinement, with a cubic cell and a small |
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| 38 | zero-point error it is possible to refine both together. Set the refinement |
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| 39 | flag for the lattice (shown above) and the zero point correction (shown below). |
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| 40 | <BR clear=left><P> |
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| 41 | <img src="fit2.gif"> |
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| 42 | <BR clear=left><P> |
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| 43 | and then start GENLES, as was done before. |
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| 44 | <BR clear=left><P> |
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| 45 | <img src="fit3.gif"> |
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| 46 | <BR clear=left> |
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| 47 | A dramatic improvement is seen both through the agreement factors (above) |
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| 48 | and as seen in the plot (below). |
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| 49 | <BR clear=left><P> |
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| 50 | <a name="ticks"><img src="fit4.gif"></a> |
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| 51 | <BR clear=left><P> |
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| 52 | At this point it is a good idea to run POWPREF and GENLES again, |
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| 53 | since peak positions have moved. This causes the Reduced Chi**2 to drop from |
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| 54 | 14.9 to 13.5. |
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| 55 | |
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| 56 | <P><BR>(.EXP Snapshot: |
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| 57 | <A HREF="ftp://ftp.ncnr.nist.gov/pub/cryst/gsas/tutorials/garnet6.exp"> |
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| 58 | garnet6.exp |
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| 59 | </A>) |
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| 60 | |
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| 61 | <BR clear=left><HR> |
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| 62 | Previous step: <A HREF="garnet5.html"> |
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| 63 | Plot data using POWPLOT or LIVEPLOT</A> |
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| 64 | </A> |
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| 65 | <BR> |
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| 66 | Next step: <A HREF="garnet7.html"> |
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| 67 | Define Constraints</A> |
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| 68 | |
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| 69 | <HR> |
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| 70 | </blockquote> |
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| 71 | <P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR> |
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| 72 | <font size=-1><? lastmod(); ?> </font> |
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| 73 | <BR>$Revision: 598 $ $Date: 2009-12-04 23:08:51 +0000 (Fri, 04 Dec 2009) $ |
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| 74 | </body> |
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| 75 | </html> |
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