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<p class="MsoNormal">Dear Myrsini,</p>
<p class="MsoNormal">A negative S220 is perhaps �improbable� and fixing it at zero (or maybe 1.0) isn�t a bad idea. The mustrain surface plot will show, by a strong indentation in the surface, the effect of a small S220. For whatever its worth, the implication
is that the structure is very �stiff� in that direction/plane. You can also try setting it to a small positive value, and then repeating the refinement. Sometimes, the least squares will push a too large initial value negative on a refinement cycle and then
not be able to return it to be above zero. Starting it small & positive then allows the LS to find a better value that is otherwise close to zero. I�ve seen this when the isotropic mustrain is apparently very small & starting at the default (1000) yields a
negative mustrain on the 1<sup>st</sup> cycle; starting at 100 yields a more sensible result.</p>
<p class="MsoNormal">Best,</p>
<p class="MsoNormal">Bob</p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Sent from <a href="https://go.microsoft.com/fwlink/?LinkId=550986">
Mail</a> for Windows</p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> GSAS-II <gsas-ii-bounces@aps.anl.gov> on behalf of Myrsini K. via GSAS-II <gsas-ii@aps.anl.gov><br>
<b>Sent:</b> Tuesday, November 1, 2022 8:31:57 AM<br>
<b>To:</b> gsas-ii@aps.anl.gov <gsas-ii@aps.anl.gov><br>
<b>Subject:</b> [GSAS-II] Generalized-Model-Negative-Parameter</font>
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<p class="x_MsoNormal"><span class="x_DefaultFontHxMailStyle">Dear GSAS-II users,</span></p>
<p class="x_MsoNormal"><span class="x_DefaultFontHxMailStyle"> </span></p>
<p class="x_MsoNormal"><span class="x_DefaultFontHxMailStyle">I am refining a synchrotron�s pattern of intercalated FeSe sample named LiPyFeSe, with Pnma space group. I�ve tried to use the
</span><span class="x_DefaultFontHxMailStyle"><b>generalized microstrain</b> model which well-fits couple of the high-Q peaks which the isotropic model doesn�t. BUT, one of the parameters (</span><span class="x_DefaultFontHxMailStyle"><u>S220</u>) of the generalized
model becomes </span><span class="x_DefaultFontHxMailStyle"><u>negative</u>. I�ve tried
</span><span class="x_DefaultFontHxMailStyle"><u>not to refine this specific parameter</u> and the pattern is well fitted again.
</span></p>
<p class="x_MsoNormal"><span class="x_DefaultFontHxMailStyle"> </span></p>
<p class="x_MsoNormal"><span class="x_DefaultFontHxMailStyle">My questions are:</span></p>
<ol start="1" type="I" style="margin-top:0in">
<li class="x_MsoListParagraph" style="margin-left:0in"><span class="x_DefaultFontHxMailStyle">What could be the meaning of the
</span><span class="x_DefaultFontHxMailStyle"><u>negative S220 parameter</u>? (note: I have calculated instrument parameters from calibrant and also I�ve investigated the family of 220 planes in the patterns and I didn�t find something weird eg. Bkgr, noise)</span></li><li class="x_MsoListParagraph" style="margin-left:0in"><span class="x_DefaultFontHxMailStyle">Is it obligatory to refine all the parameters of the generalized model, coming from the selected space group?</span></li></ol>
<p class="x_MsoListParagraph"><span class="x_DefaultFontHxMailStyle"> </span></p>
<p class="x_MsoNormal"><span class="x_DefaultFontHxMailStyle">Thank you in advance!</span></p>
<p class="x_MsoNormal"><span class="x_DefaultFontHxMailStyle"> </span></p>
<p class="x_MsoNormal">Best regards,</p>
<p class="x_MsoNormal">KAITATZI Myrsini</p>
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