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<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Dear Christopher,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">1)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Can RMCProfile fit x-ray PDF?: Yes<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D">What Chris Kerr says is not strictly true. RMCProfile can fit x-ray PDF data.
<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D">The x-ray F(Q) can be fitted with the XRAY_RECIPROCAL_DATA keyword. However, the x-ray form factors have to be in the data file and it has to be convolved with the simulation box size. There is a program
call xray_to_rmc that will do this for you if supplied the data in the usual rmc format.<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D">For the x-ray PDF, since the q-dependant part has been divided out you can treat this as a neutron PDF with different weights. These weightings are defined using the keyword NEUTRON_COEFFICIENTS (sorry
a bit confusing I know).<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D">We are writing a tutorial example to show how all this works and it will be in the minor update 6.5.2. This will be out in the next few weeks.<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D">As Chris Kerr says there are plans to fit the x-ray PDF in a more rigorous way. Once this is implemented it will be useful for all x-ray PDF data, as it will make a fairer comparison regardless of the
atomic radii<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">2)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">How do you fit nanoparticles with RMCProfile?<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph"><span style="color:#1F497D">Sadly this is not an easy question to answer, there are three possible approaches.
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo2">
<![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">a.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">The first is to treat the data as a bulk sample if the particle is large. If not then a modification function for the particle size could be applied but this isn’t implemented at present.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo2">
<![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">b.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">The second (as Chris says) is to put one particle in a large box and only fit out to a distance where it does not see its periodic repeat (RMC uses periodic boundary conditions).
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo2">
<![if !supportLists]><span style="color:#1F497D"><span style="mso-list:Ignore">c.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="color:#1F497D">Finally if you can build a starting configuration with particles in a matrix that is not too large (> ~40,000 atoms), then you can use the various constraints/restraints to hold things together and
model it as normal. This of course is not trivial but probably the ideal solution.
<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:54.0pt"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:54.0pt"><span style="color:#1F497D">If you tell us some more about your system we can advise what your best approach is most likely to be.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Best wishes,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Matt<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> rmcprofile-users-bounces@rmcprofile.org [mailto:rmcprofile-users-bounces@rmcprofile.org]
<b>On Behalf Of </b>Christopher J. Wright<br>
<b>Sent:</b> 02 December 2013 00:36<br>
<b>To:</b> rmcprofile-users@rmcprofile.org<br>
<b>Subject:</b> [RMCProfile-users] X-ray PDF use<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I am working with PDF data collected from an x-ray synchrotron source. The samples are nanoparticles of various size and compositions. Is it possible to model these particles using RMCprofile?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thank you very much,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Christopher J. Wright<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Brown University Chemistry Department<o:p></o:p></span></p>
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