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<div>Dear Gabriele,</div>
<div>yes, I changed the volume, it's less then the relaxed value. this way TO phonon is stable at gamma point. </div>
<div>the right values for TA branch should go to 0.</div>
<div>Alexander</div>
<div><br>
Le 20 août 2014 à 10:45, "Gabriele Sclauzero" <<a href="mailto:gabriele.sclauzero@mat.ethz.ch">gabriele.sclauzero@mat.ethz.ch</a>> a écrit :<br>
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<div class="moz-cite-prefix">Dear Alexander,<br>
<br>
<br>
On 08/19/2014 05:08 PM, Kvasov Alexander wrote:<br>
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black">Dear developers,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black">I study flexoelectric effect which can be seen as a repulsion of TA and TO branches in the vicinity of gamma point. I calculate phonon dispersion of cubic SrTiO3.
It’s essential for me to have adequate picture of phonons in the vicinity of gamma point (at least for q>0.05).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black">The picture shows TA and TO branches single q-point (st.ph.single.in) calculations (grey dots) and dispersion (st.ph.disp.in) for 8x8x8 mesh.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black">With used parameters ph.x fails to give right values for small values of q-vector.
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What are the "right values" you were expecting? Probably you know this already, but within PBE (probably also LDA) SrTiO3 displays a ferroelectric instability at the theoretical lattice constant, which triggers the imaginary frequency at Gamma. This is an artifact
of the approximate functional. You might get rid of that by changing the volume (I don't remember in which direction, hopefully toward the experimental value...), or using some other functional.<br>
<br>
<br>
<br>
HTH<br>
<br>
GS<br>
<br>
<br>
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<blockquote cite="mid:1CD6B4F59146514CAC7DF7BB8C48DDD157C5D8CC@rexma.intranet.epfl.ch" type="cite">
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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black">I guess it’s due to numerical errors of the code.<o:p></o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black">What are the main reasons for this numerical error when we go close to gamma point. Which parameters should I adjust to improve the situation?<o:p></o:p></span></b></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black">Attached, all relevant inputs and outputs<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Cambria","serif";color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">Sincerely yours,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">Alexander Kvasov<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">Ceramics Laboratory /
</span> <span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">THEOS (Theory and Simulation of Materials) Laboratory<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB" lang="FR-CH">École Polytechnique Fédérale de Lausanne (EPFL)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB" lang="FR-CH"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">Phone: +41 21 693 5805<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">Address:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">EPFL STI IMX LC<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">MXD 220 (Bâtiment MXD) Station 12<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Cambria","serif";color:black;mso-fareast-language:EN-GB">1015 Lausanne, Switzerland<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="FR-CH"><o:p> </o:p></span></p>
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<pre class="moz-signature" cols="72">--
Dr. Gabriele Sclauzero
ETH Zurich
Materials Theory
HIT G 43.2
Wolfgang-Pauli-Str. 27
8093 Zürich, Switzerland
Phone +41 44 633 94 10
Fax +41 44 633 14 59
<a class="moz-txt-link-abbreviated" href="mailto:gabriele.sclauzero@mat.ethz.ch">gabriele.sclauzero@mat.ethz.ch</a>
<a class="moz-txt-link-freetext" href="http://www.theory.mat.ethz.ch/people/postdocs/gsclauze">http://www.theory.mat.ethz.ch/people/postdocs/gsclauze</a></pre>
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