[QE-users] non-convergent scf calculation on organometal perovskite structure
Julien Barbaud
julien_barbaud at sjtu.edu.cn
Thu Feb 28 13:27:04 CET 2019
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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>>>
>>>
>>>
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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= 65, ecutrho = 500
vdw_corr='xdm', xdm_a1=0.0000, xdm_a2=4.1503
/
&ELECTRONS
mixing_beta = 0.001
conv_thr = 1.0d-6
electron_maxstep = 500
/
&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
6.30000000 0.00000000 0.00000000
0.00000000 6.30000000 0.00000000
0.00000000 0.00000000 6.30000000
ATOMIC_POSITIONS angstrom
I 3.15000000 3.15000000 0.00000000
I 0.00000000 3.15000000 3.15000000
I 3.15000000 0.00000000 3.15000000
Pb 3.15000000 3.15000000 3.15000000
N -0.44740000 -0.44740000 -0.44740000
C 0.44740000 0.44740000 0.44740000
H 0.26870000 0.26870000 1.37360000
H 1.37360000 0.26870000 0.26870000
H 0.26870000 1.37360000 0.26870000
H -0.28180000 -1.30600000 -0.28180000
H -0.28180000 -0.28180000 -1.30600000
H -1.30600000 -0.28180000 -0.28180000
K_POINTS automatic
4 4 4 0 0 0
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