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

[Pietro Delugas]

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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