Dear Fabrizio,
Thank you for your very quick answer and your suggestions. To answer
your question, I did not inlcude smearing in my most general attempts
(the "by default" input file), because it was my understanding that
semiconductors /should/ not require it. Indeed, the charge density
already decays smoothly to 0 around the band gap, so it should not pose
the numerical problems that metals do. Am I mistaken ?
However, as mentionned, I did experiment with a fix where I include some
additional empty bands to see if that solves the problem (as suggested
in: https://www.quantum-espresso.org/Doc/pw_user_guide/node21.html
section5.0.0.21). In this case, I used a Methfessel-Paxton smearing
(m-p). I tried 30 bands (instead of 25 by default) with several values
of broadening, but this does not appear to solve the problem.
As shown in the figure attached (reporting the evolution of estimated
accuracy at each scf iteration), there is no obvious advantage in
raising the value of degauss. The curve still stalls after a cetain
number of iterations.
I also attached the input file used for this test.
Julien
Le 21/02/2019 à 12:53, Fabrizio Cossu a écrit :
Dear Julien,
what smearing did you have in mind? Have you tried raising the broadening?
Cheers,
Fabrizio
On Thu, 21 Feb 2019 at 12:19, Julien Barbaud
<[email protected] <mailto:[email protected]>> 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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--
*Fabrizio Cossu
postdoctoral fellow at APCTP (Asia Pacific Center for Theoretical
Physics)*,
Hogil Kim Memorial Building #501
POSTECH, 67 Cheongam-Ro, Nam-Gu,
Pohang-si, Gyeongsangbuk-do,
790-784 (37673), Republic of Korea
|
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&CONTROL
pseudo_dir = "/home/julien_barbaud/QUANTUM_ESPRESSO/Program/qe-6.3/pseudo"
calculation = 'scf'
outdir= "./out"
tstress=.TRUE.
tprnfor=.TRUE.
/
&SYSTEM
nat= 12
ntyp= 5
ibrav= 0
ecutwfc= 30, ecutrho = 300
nbnd= 30
occupations= 'smearing', smearing = 'm-p', degauss = 0.1
/
&ELECTRONS
mixing_beta = 0.00075
conv_thr = 1.0d-8
electron_maxstep = 70
/
&IONS
/
&CELL
/
ATOMIC_SPECIES
C 12.011 C.pbesol-n-rrkjus_ps1.1.0.0.UPF
N 14.007 N.pbesol-n-rrkjus_ps1.1.0.0.UPF
H 1.008 H.pbesol-rrkjus_ps1.0.1.UPF
Pb 207.2 Pb.pbesol-dn-rrkjus_ps1.1.0.0.UPF
I 126.90 I.pbesol-n-rrkjus_ps1.1.0.0.UPF
CELL_PARAMETERS angstrom
6.28880000 0.00000000 0.00000000
0.00154263 6.22876981 0.00000000
0.13973295 -0.00036613 6.37255819
ATOMIC_POSITIONS angstrom
C 5.68762816 6.22746232 2.91755469
N 0.76248596 -0.00015695 3.49068983
H 5.76522586 6.22643528 1.82113056
H 5.15723272 0.90114080 3.25657478
H 5.15968675 5.32511200 3.25829538
H 1.30458379 0.84536565 3.19592078
H 1.30663813 5.38278144 3.19763499
H 0.76813187 0.00090438 4.53227809
Pb 2.93516392 3.11390239 6.22995946
I 2.64234654 3.11409882 3.02788916
I 2.51578467 6.22849954 0.15080022
I 5.99299067 3.11401602 5.87855111
K_POINTS automatic
4 4 4 0 0 0
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