[QE-users] non-convergent scf calculation on organometal perovskite structure

Julien Barbaud julien_barbaud at sjtu.edu.cn
Fri Feb 22 08:22:05 CET 2019


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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>> users at lists.quantum-espresso.org
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>
>
>
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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
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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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