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<p>Thanks very much to everyone for their answers,</p>
<p>I have tried the different fixes suggested (higher ecuts, trying
out different smearings, raising mixing_ndim, changing the
mixing_mode, etc...). Unfortunatley, none of them worked...<br>
</p>
<p>Following Pietro's recommandation, I also designed an input
geometry perfectly cubic with no distortion and the MA+ ion
oriented along the [1 1 1] direction for higher symmetry. I based
the quantitative part of the geometry on other cif files available
(i.e. for the lattice constant, bond lengths, etc...)</p>
<p>Benefitting from the high symmetry, the algorithm worked faster,
but was still unable to converge to a threshold of 1e-7Ry in 500
iterations...</p>
<p><br>
</p>
<p>I finally relaxed the condition to 1e-6Ry in order to reach a scf
convergence, and see if I could get more informations on the
problem from it. <br>
</p>
<p>Under those conditions I plotted the scf total energy (converged
to 1e-6Ry) as a function of ecut. I got the curve attached. This
seems surprising to me because the curve is raising past 45Ry<br>
</p>
<p>I thought that the total energy would only have a monotonic
trend, decreasing with higher ecut, as a consequence of the
variational principle (indeed, the energy of the ground state
should be a minimum for all possible energies, so a truncated
version of the ground state wavefunction should yield a higher
value of energy). Here it seems to break that rule, and to
"converge" to a value that is not the minimum among all possible
wavefunctions. This can not be attributed to the accuracy of the
calculation, because the scf is converged down to 1e-6Ry, and the
augmentation observed is significantly above that threshold<br>
</p>
<p>Did I misunderstand something, or is it another sign that
something is seriously wrong with the calculation ?</p>
<p><br>
</p>
<p>(by the way, the curve stops after 65Ry, because scf failed to
reach convergence again at ec=70Ry, even for a threshold at 1e-6
and 500 iterations).</p>
<p>What other options should I try to solve this problem ?</p>
<p><br>
</p>
<p>Julien<br>
</p>
<p><br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">Le 22/02/2019 à 16:45, Pietro Delugas a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:0244887b-b39c-55c3-471a-2a4df142d68d@sissa.it">
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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><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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charset=UTF-8">
<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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