Hi Julien
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.
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.
greetings Pietro
On 03/08/2019 04:34 AM, Julien Barbaud wrote:
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.
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.
Thanks again for the help.
Julien
Le 01/03/2019 à 11:31, Pietro Davide Delugas a écrit :
Hi Julien
I checked your input, and N.pbesol-n-kjpaw_psl.1.0.0.UPF
<http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.1.0.0.UPF>
seems to be the cause of your problems, you can use this one instead
http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.0.1.UPF
and you should have no more problems with negative charges and have
a more or less fast convergence.
Pietro
On 02/28/2019 01:27 PM, Julien Barbaud wrote:
Thanks very much to everyone for their answers,
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...
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...)
Benefitting from the high symmetry, the algorithm worked faster, but
was still unable to converge to a threshold of 1e-7Ry in 500
iterations...
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.
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
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
Did I misunderstand something, or is it another sign that something
is seriously wrong with the calculation ?
(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).
What other options should I try to solve this problem ?
Julien
Le 22/02/2019 à 16:45, Pietro Delugas a écrit :
Dear Julien
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.
obviously before worrying about accuracy you would like to have a
converged density.
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.
On 22/02/19 08:22, Julien Barbaud wrote:
Thank you Pietro for your experienced advices,
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
(https://aip.scitation.org/doi/full/10.1063/1.4864778), or even
single gamma-point calculation
(http://people.bath.ac.uk/aw558/publications/2013/aplm_perovskite_13.pdf),
so I guess this should not be the obstacle to convergence here.
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 ?
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
Julien
Le 21/02/2019 à 16:35, Pietro Davide Delugas a écrit :
Hi
Have you tried to increase the k_point mesh ? 4 4 4 seems a
little bit lax as mesh for MAPbI3.
If I remember well I am afraid that to get convergence you will
need something like 10X10X10.
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 (
https://www.quantum-espresso.org/Doc/INPUT_PW.html#idm45922794348896)
hope it helps
Pietro
On 02/21/2019 04:17 AM, Julien Barbaud wrote:
Dear users,
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.
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:
* 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)
* varying cutoff energy for charge (cf ecutrho.png)
* taking larger k-point sampling (not shown)
* 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)
The only change that I found effective was to reduce the
mixing_beta factor.
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.
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.
Any insight on what I did wrong would be really helpful. I
suspect a shameful beginner mistake, but can not find it out.
Thanks in advance,
Julien barbaud
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 !
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