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<div class="moz-cite-prefix">Hi Julien <br>
<br>
Nicola Marzari in the email before explained what was going
wrong, either that was because of too low cutoffs or because of
some problems in the PAWS, as cutoffs looked right I wanted to
check the PPs. To spot the bad one you have just to change them
one per time. <br>
<br>
In the test I did, I got convergence in less than 20 iterations,
perhaps you are still using a too small value for mixing_beta.
<br>
<br>
greetings Pietro <br>
<br>
On 03/08/2019 04:34 AM, Julien Barbaud wrote:<br>
</div>
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cite="mid:0f7a49e8-2e42-9b80-d55a-aa63d51e2ea6@sjtu.edu.cn">
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<p> Thank you very much Pietro, this actually fixed the
issue ! The frustrating stalling behaviour in the
convergence has been suppressed, and I can now reach
convergence within 1e-8Ry in less than 100 iterations.<br>
</p>
<p><br>
</p>
<p> If I am not abusing of your time, can I ask you how you
spotted that this particular PP was the issue? Gaining some
insight on what was going wrong or how to debug those
problems would be very benefitial for me.</p>
<p><br>
</p>
<p>Thanks again for the help.</p>
<p>Julien<br>
</p>
<div class="moz-cite-prefix">Le 01/03/2019 à 11:31, Pietro
Davide Delugas a écrit :<br>
</div>
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cite="mid:54b911fe-f433-8186-ca49-c3ef13286fa3@sissa.it">
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<div class="moz-cite-prefix">Hi Julien <br>
<br>
I checked your input, and <a class="element_anchor"
target="_blank"
href="http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.1.0.0.UPF"
moz-do-not-send="true">N.pbesol-n-kjpaw_psl.1.0.0.UPF</a>
seems to be the cause of your problems, you can use this
one instead <br>
<a class="moz-txt-link-freetext"
href="http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.0.1.UPF"
moz-do-not-send="true">http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.0.1.UPF</a>
<br>
<br>
and you should have no more problems with negative
charges and have a more or less fast convergence. <br>
Pietro <br>
<br>
On 02/28/2019 01:27 PM, Julien Barbaud wrote:<br>
</div>
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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>
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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>
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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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<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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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>
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