<div dir="ltr"><div dir="ltr"> Dear Julien,<div>I'd go for the suggestion proposed by Pietro. Mind also that, for semiconductor, Methfessel-Paxton is not appropriate, as far as I know. Keep Gaussian or try Fermi smearing. What's the recommended value for the cutoff for each pseudopotential? An insufficient cutoff may considerably affect the results, especially for insulators (forgive me for having overlooked the electronic character of the compound previously). Moreover, the theoretical lattice constant may be substantially different from the experimental one; a very different lattice constant more likely gives rise to numerical instabilities.</div><div><br></div><div> Best wishes,</div><div> Fabrizio</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, 22 Feb 2019 at 15:50, Julien Barbaud <<a href="mailto:julien_barbaud@sjtu.edu.cn">julien_barbaud@sjtu.edu.cn</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p>Dear Fabrizio,</p>
<p>Thank you for your very quick answer and your suggestions. To
answer your question, I did not inlcude smearing in my most
general attempts (the "by default" input file), because it was my
understanding that semiconductors <i>should</i> not require it.
Indeed, the charge density already decays smoothly to 0 around the
band gap, so it should not pose the numerical problems that metals
do. Am I mistaken ?</p>
<p><br>
</p>
<p>However, as mentionned, I did experiment with a fix where I
include some additional empty bands to see if that solves the
problem (as suggested in:
<a class="gmail-m_-5586545909898028533moz-txt-link-freetext" href="https://www.quantum-espresso.org/Doc/pw_user_guide/node21.html" target="_blank">https://www.quantum-espresso.org/Doc/pw_user_guide/node21.html</a>
section<a name="m_-5586545909898028533_SECTION000600210000000000000" style="color:rgb(0,0,0);font-family:"Microsoft YaHei";font-size:16px;font-style:normal;font-variant:normal;letter-spacing:normal;text-align:left;text-decoration:none;text-indent:0px;text-transform:none;white-space:normal;word-spacing:0px">5.0.0.21)</a>. In this
case, I used a Methfessel-Paxton smearing (m-p). I tried 30 bands
(instead of 25 by default) with several values of broadening, but
this does not appear to solve the problem. <br>
</p>
<p>As shown in the figure attached (reporting the evolution of
estimated accuracy at each scf iteration), there is no obvious
advantage in raising the value of degauss. The curve still stalls
after a cetain number of iterations. <br>
</p>
<p>I also attached the input file used for this test.<br>
</p>
<p><br>
</p>
<p>Julien<br>
</p>
<p><br>
</p>
<div class="gmail-m_-5586545909898028533moz-cite-prefix">Le 21/02/2019 à 12:53, Fabrizio Cossu a
écrit :<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr"> Dear Julien,
<div>what smearing did you have in mind? Have you tried
raising the broadening?</div>
<div><br>
</div>
<div> Cheers,</div>
<div> Fabrizio</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Thu, 21 Feb 2019 at
12:19, Julien Barbaud <<a href="mailto:julien_barbaud@sjtu.edu.cn" target="_blank">julien_barbaud@sjtu.edu.cn</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div bgcolor="#FFFFFF"> Dear users,
<div class="gmail-m_-5586545909898028533gmail-m_-3410636194943835992moz-forward-container">
<div class="gmail-m_-5586545909898028533gmail-m_-3410636194943835992moz-forward-container">
<p><br>
</p>
<p>I am new to QE, and trying to run a simple scf
calculation on a CH3NH3PbI3 crystal (semi-conducting
material). I am using ultrasoft pseudopotentials
based on the exchange-correlation functionnal
PBEsol.<br>
</p>
<p> I set up a first input, with values of parameters
inspired from literature on the subject. However, I
could not reach convergence after 100 iterations.
The estimated error was actually "exploding" to very
high values, indicating a serious problem. I tried
several changes but was unsuccessful: <br>
</p>
<ul>
<li> varying plane-wave cutoff energy does not solve
the problem (cf attached ecut.png, giving the
estimated error as a function of the number of
iterations. It is shown here only on the first 15
iterations as the results pretty much only stall
from there)</li>
<li> varying cutoff energy for charge (cf
ecutrho.png)</li>
<li>taking larger k-point sampling (not shown)</li>
<li> I also read that for metallic or "close to
metallic conductors", there might be problems with
the first unoccupied states that can be solved by
adding a few empty bands. My system being a
semi-conductor, I tried adding additional bands
using a m-p smearing but no improvement was found
(not shown)</li>
</ul>
<p><br>
</p>
<p>The only change that I found effective was to
reduce the mixing_beta factor.<br>
</p>
<p><br>
</p>
<p>It effectively prevents the error from diverging to
very large values, but I still do not reach
convergence, even after longer iterations. I tried
much smaller values of mixing beta which improves
the final value of the error, but I still cannot
reach convergence on 100 iterations. As shown in the
mixbeta2_zoom.png, the error reduces to smaller
values around ~1e-5~1e-6, but it keeps stalling
after a while. I do not observe a well-converging
behaviour for any value.</p>
<p><br>
</p>
<p>I attached the "default version" of my script on
which the various modifications described above have
been independently performed. I obtained the
geometry from a CIF file in literature and checked
it with visualization software; it seems perfectly
ok as far as I can tell.<br>
</p>
<p><br>
</p>
<p>Any insight on what I did wrong would be really
helpful. I suspect a shameful beginner mistake, but
can not find it out.<br>
</p>
<p><br>
</p>
<p>Thanks in advance,</p>
<p>Julien barbaud</p>
<p><br>
</p>
<p>P.S: this is my first time posting on this user
list. Please let me know if my question is not
suitable for it, or can be improved either in its
content or presentation. I will gladly take any
recommandation into account in order not to
negatively impact the quality of this user list !<br>
</p>
</div>
</div>
</div>
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-- <br>
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<div dir="ltr">
<div dir="ltr">
<div dir="ltr"><b>Fabrizio Cossu<br>
postdoctoral fellow at APCTP (Asia
Pacific Center for Theoretical
Physics)</b>,</div>
<div dir="ltr"><font face="comic sans
ms, sans-serif">Hogil Kim Memorial
Building #501 <br>
POSTECH, 67 Cheongam-Ro, Nam-Gu, <br>
Pohang-si, Gyeongsangbuk-do,<br>
790-784 (37673), Republic of Korea</font></div>
<div dir="ltr">
<pre> |
.----. .----. .----. |---- .----. ===========================
,----| | | | | | | <a href="http://www.apctp.org/?JrgId=16" style="font-family:"comic sans ms",sans-serif;white-space:normal;color:rgb(17,85,204)" target="_blank">http://www.apctp.org/?JrgId=16</a>
`----^ |----' `----' `----' |----' ===========================
| |</pre>
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