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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
(<a class="moz-txt-link-freetext" href="https://aip.scitation.org/doi/full/10.1063/1.4864778">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">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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<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>
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cite="mid:a55d199b-54b2-09be-4cc5-9826e0151a09@sjtu.edu.cn">
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Dear users,
<div class="moz-forward-container">
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<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>
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