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<p><font size="-1">Dear Julien <br>
</font></p>
<p><font size="-1">even if the scf loop converges you have still to
check that the k-point sampling and the plane wave basis set
guarantee you an accurate result. <br>
</font></p>
<p><font size="-1"> obviously before worrying about accuracy you
would like to have a converged density. <br>
</font></p>
<p><font size="-1">You could try to start with a more symmetric
cell, use a cubic cell without distortions and align the
molecule along one of the diagonals of the perovskite box. <br>
</font></p>
<p><font size="-1"></font><br>
</p>
<div class="moz-cite-prefix">On 22/02/19 08:22, Julien Barbaud
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:5ed6547f-f86e-5f57-ea86-06805e1a99fd@sjtu.edu.cn">
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<p> </p>
<div class="moz-forward-container">
<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
(<a class="moz-txt-link-freetext"
href="https://aip.scitation.org/doi/full/10.1063/1.4864778"
moz-do-not-send="true">https://aip.scitation.org/doi/full/10.1063/1.4864778</a>),
or even single gamma-point calculation
(<a class="moz-txt-link-freetext"
href="http://people.bath.ac.uk/aw558/publications/2013/aplm_perovskite_13.pdf"
moz-do-not-send="true">http://people.bath.ac.uk/aw558/publications/2013/aplm_perovskite_13.pdf</a>),
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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charset=UTF-8">
<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>
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