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

Julien Barbaud julien_barbaud at sjtu.edu.cn
Fri Mar 8 04:34:43 CET 2019


     Thank you very much Pietro, this actually fixed the issue ! The 
frustrating stalling behaviour in the convergence has been suppressed, 
and I can now reach convergence within 1e-8Ry in less than 100 iterations.


     If I am not abusing of your time, can I ask you how you spotted 
that this particular PP was the issue? Gaining some insight on what was 
going wrong or how to debug those problems would be very benefitial for me.


Thanks again for the help.

Julien

Le 01/03/2019 à 11:31, Pietro Davide Delugas a écrit :
> Hi Julien
>
> I checked your input, and N.pbesol-n-kjpaw_psl.1.0.0.UPF 
> <http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.1.0.0.UPF> 
> seems to be the cause of your problems, you can use   this one instead
> http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.0.1.UPF
>
> and you should  have no more problems  with negative charges and have 
> a more or less fast convergence.
>     Pietro
>
> On 02/28/2019 01:27 PM, Julien Barbaud wrote:
>>
>> 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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