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

Pietro Davide Delugas pdelugas at sissa.it
Fri Mar 1 11:31:24 CET 2019 

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
<http://www.quantum-espresso.org/upf_files/N.pbesol-n-kjpaw_psl.1.0.0.UPF>
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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>>>>
>>>>
>>>>
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