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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>
</div>
<blockquote
cite="mid:1CD6B4F59146514CAC7DF7BB8C48DDD157C5D8CC@rexma.intranet.epfl.ch"
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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. </span></p>
</div>
</blockquote>
<br>
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>
<br>
<blockquote
cite="mid:1CD6B4F59146514CAC7DF7BB8C48DDD157C5D8CC@rexma.intranet.epfl.ch"
type="cite">
<div class="WordSection1">
<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>
</div>
</blockquote>
<br>
<br>
<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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