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| 2 | <head> |
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| 3 | <title>EXPGUI Alumina Tutorial, part #4</title> |
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| 11 | <BODY BGCOLOR="#FFFFFF" |
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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 | <TABLE BORDER BGCOLOR="#FFFF40" ALIGN=RIGHT> |
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| 22 | <TR><TH><A Href="index.html">Intro</A> |
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| 23 | </TH><TH><A Href="alumina5.html">Next page</A> |
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| 24 | </TH><TH><A Href="alumina3.html">Previous page</A> |
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| 25 | </TH></TR></TABLE><BR CLEAR=ALL> |
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| 26 | |
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| 27 | |
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| 28 | <CENTER> |
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| 29 | <H1><HR noshade width="75%" size="2" align="center"> |
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| 30 | GSAS/EXPGUI Alumina tutorial (part 4)<BR> |
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| 31 | Changing the Background Function |
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| 32 | <HR noshade width="75%" size="2" align="center"> |
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| 33 | </H1> |
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| 34 | </CENTER> |
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| 35 | <I>GSAS offers approximately 10 different background functions (not all are |
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| 36 | implemented in EXPGUI). For each of these functions, the number of terms |
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| 37 | to be used is adjustable. The more terms the more complex the shape that |
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| 38 | can be fit. |
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| 39 | Each of these background functions |
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| 40 | has different shapes, and in theory, each function will have advantages |
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| 41 | under different circumstances. |
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| 42 | However, this author |
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| 43 | finds that the Shifted Chebyschev (type #1) is preferable to the |
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| 44 | others for the vast majority of Rietveld refinements and almost never uses |
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| 45 | any other function.</I> |
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| 46 | <P> |
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| 47 | When a histogram is first added in GSAS, the background function is set to the |
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| 48 | Cosine Fourier series option (type #2) with two adjustable terms. |
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| 49 | In this section of the tutorial, we change the background function and the |
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| 50 | number of terms. |
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| 51 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 52 | To change the background function, press the "Edit Background" button on the |
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| 53 | histogram panel. |
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| 54 | <P> |
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| 55 | <img alt="tutorial screen dump" src="4a.gif" align=RIGHT> |
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| 56 | <BR clear=all> |
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| 57 | <P><blockquote><I> |
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| 58 | Note the "Refine background" check box has been selected -- this means that |
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| 59 | the background parameters will be refined (optimized) when GENLES is run. |
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| 60 | The damping parameter to the immediate left is set to 0 -- this means that |
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| 61 | the full computed shift will be applied. In cases where a refinement has |
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| 62 | trouble reaching a minimum, it may be advantageous to increase damping |
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| 63 | (a setting of 1 implies 90% of the computed shift will be applied and |
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| 64 | a damping setting of 9 yields a shift of 10%.) |
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| 65 | <P> |
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| 66 | Every refinable parameter in |
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| 67 | GSAS has a refinement flag (either for the group, as in this case, |
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| 68 | or for each individual parameter) and a damping parameter. The appearance |
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| 69 | of the check box is slightly different in windows: a "x" appears in the |
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| 70 | box when the parameter is selected. |
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| 71 | </I></blockquote> |
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| 72 | |
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| 73 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 74 | <img alt="tutorial screen dump" src="4b.gif" align=RIGHT> |
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| 75 | After the "Edit Background" button is pressed, |
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| 76 | The "Edit Background" window opens, as is shown to the right. |
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| 77 | In this window both the function type and the |
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| 78 | number of terms can be changed. |
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| 79 | <BR clear=all> |
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| 80 | <img alt="tutorial screen dump" src="4c.gif" align=RIGHT> |
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| 81 | Clicking on the "Function type" menu offers a choice of function types, |
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| 82 | as is shown to the right. Choose function type #1, the Shifted |
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| 83 | Chebyschev function. |
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| 84 | <BR clear=all> |
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| 85 | <P><blockquote><I> |
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| 86 | Note that the "Fit Background Graphically" invokes the EXPGUI |
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| 87 | <a href="http://www.ncnr.nist.gov/xtal/software/liveplot.html#bkgedit"> |
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| 88 | BKGEDIT</A> program, which is used to fit a background function |
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| 89 | directly to user-supplied points. This is a very useful initial way to |
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| 90 | fit the background in difficult cases. |
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| 91 | </I></blockquote> |
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| 92 | <P><HR noshade width="50%" size="4" align="center"><P> |
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| 93 | <img alt="tutorial screen dump" src="4d.gif" align=RIGHT> |
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| 94 | Also in the "Edit Background" window, press on the "Number of terms" button |
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| 95 | and change this number to 6, as is shown to the right. Note that with |
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| 96 | the Chebyschev polynomial arbitrary (including zero) starting values for the |
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| 97 | background terms is acceptable. |
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| 98 | <P> |
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| 99 | After the "Set" button is pressed, the changes are then seen on the histogram |
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| 100 | panel, as shown below. |
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| 101 | Note that by default, the refine background flag is set, allowing these |
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| 102 | parameters to be optimized. |
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| 103 | <BR clear=all><P> |
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| 104 | <img alt="tutorial screen dump" src="4e.gif" align=RIGHT> |
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| 105 | <BR clear=all> |
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| 106 | <HR> |
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| 107 | Previous: <A HREF="alumina3.html"> |
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| 108 | Specifying Powder Diffraction Data |
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| 109 | </A> |
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| 110 | <BR> |
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| 111 | Next step: <A HREF="alumina5.html"> |
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| 112 | Initial Fitting: Refine Scale Factor and Background |
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| 113 | </A> |
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| 114 | |
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| 115 | <HR> |
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| 116 | </blockquote> |
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| 117 | <P><font size=-1><A HREF="MAILTO:crystal@NIST.gov?subject=WWW page <?=$PHP_SELF?>">Comments, corrections or questions: crystal@NIST.gov</A></font><BR> |
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| 118 | <font size=-1><? lastmod(); ?> |
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| 119 | <BR>$Revision: 623 $ $Date: 2009-12-04 23:09:16 +0000 (Fri, 04 Dec 2009) $</font> |
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| 120 | </body> |
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| 121 | </html> |
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