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<p>Thank you Pietro for your experienced advices,</p>
<p><br>
</p>
<p>I had tried to increase the kmesh size before but only up to
sizes of 7x7x7. Reading your suggestions, I ran additional tests
up to 10x10x10 but this did not show any sign of improvement on 70
iterations. As shown in file kmesh.png, the estimated accuracy is
still stagnating after a while and the 10*10*10 is actually giving
arguably worse results than the 9*9*9 although this is most likely
not significant. Actually, some papers report DFT simulation of
MAPbI3 using 6x6x6 kmesh, or even single gamma-point calculation,
so I guess this should not be the obstacle to convergence here.</p>
<p><br>
</p>
<p>Regarding the orientation of MA, I definitely agree with you, but
I don't think it can prevent the system from converging ? Sure
enough, it can have an important influence on the precision of the
results in later uses. But I would like to achieve convergence on
this simple single cell first, before building up supercells to
take more complex effects into account. A crystal with perfectly
aligned MA might not reflect the true experimental system, but it
should still be a possible configuration that the QE code should
be able to compute, am I wrong ?</p>
<p><br>
</p>
<p>As to your suggestion on VdW corrections, I just gave it a try,
but unfortunately, this is unconclusive too. I report the accuracy
at each iteration in vdw.png. Again, the accuracy stops improving
after a while. Plese note that I had to change my
pseudo-potentials to use 'xdm' correction (which only supports PAW
PP). the input file for this test is included as attached file</p>
<p><br>
</p>
<p>Julien<br>
</p>
<div class="moz-cite-prefix">Le 21/02/2019 à 16:35, Pietro Davide
Delugas a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:fb09ead9-e310-4142-0c01-7c8af7a8ab46@sissa.it">
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<div class="moz-cite-prefix">Hi <br>
<br>
Have you tried to increase the k_point mesh ? 4 4 4 seems a
little bit lax as mesh for MAPbI3. <br>
If I remember well I am afraid that to get convergence you will
need something like 10X10X10.<br>
As for the structure neighboring methylammoniums like to orient
differently one from the other, you should probably use a larger
cell. Also consider to add some correction for van der Waals
interactions see here ( <a class="moz-txt-link-freetext"
href="https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm45922794348896"
moz-do-not-send="true">https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm45922794348896</a>)<br>
<br>
hope it helps <br>
Pietro <br>
<br>
<br>
On 02/21/2019 04:17 AM, Julien Barbaud wrote:<br>
</div>
<blockquote type="cite"
cite="mid:a55d199b-54b2-09be-4cc5-9826e0151a09@sjtu.edu.cn">
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charset=UTF-8">
Dear users,
<div class="moz-forward-container">
<div class="moz-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>
<br>
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