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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>
<blockquote type="cite"
cite="mid:54b911fe-f433-8186-ca49-c3ef13286fa3@sissa.it">
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charset=UTF-8">
<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>
<blockquote type="cite"
cite="mid:82632677-2321-3397-a229-16e00eb191af@sjtu.edu.cn">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8">
<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">
<meta http-equiv="Content-Type" content="text/html;
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">
<meta http-equiv="content-type"
content="text/html; 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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