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, or even single
gamma-point calculation, 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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&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= 45, ecutrho = 450
vdw_corr='xdm', xdm_a1=0.0000, xdm_a2=4.1503
/
&ELECTRONS
mixing_beta = 0.00075
conv_thr = 1.0d-7
electron_maxstep = 100
/
&IONS
/
&CELL
/
ATOMIC_SPECIES
C 12.011 C.pbesol-n-kjpaw_psl.1.0.0.UPF
N 14.007 N.pbesol-n-kjpaw_psl.1.0.0.UPF
H 1.008 H.pbesol-kjpaw_psl.0.1.UPF
Pb 207.2 Pb.pbesol-dn-kjpaw_psl.1.0.0.UPF
I 126.90 I.pbesol-n-kjpaw_psl.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
&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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