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

[Julien Barbaud]

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= 30, ecutrho = 300
/
 
&ELECTRONS
  mixing_beta = 0.7
  conv_thr =  1.0d-8
/
 
&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