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

Fri Feb 22 10:40:06 CET 2019

Dear Julien,

what always helped me to overcome scf convergence issues was the guideline
in the User's Guide:

"Reduce mixing beta from the default value (0.7) to ∼ 0.3 −
0.1 or smaller. Try the mixing mode value that is more appropriate for
your problem. For slab geometries used in surface problems or for
elongated cells, mixing mode=’local-TF’ should be the better choice,
dampening “charge sloshing”. You may also try to increase mixing ndim to
more than 8 (default value). Beware: the larger mixing ndim, the larger
the amount of memory you need.

If the above doesn’t help: verify if your system is metallic or is close
to a metallic state, especially if you have few k-points. If the highest
occupied and lowest unoccupied state(s) keep exchanging place during
self-consistency, forget about reaching convergence. A typical sign of
such behavior is that the self-consistency error goes down, down, down,
than all of a sudden up again, and so on. Usually one can solve the
problem by adding a few empty bands and a broadening.

Speciﬁc to US PP: the presence of negative charge density regions
due to either the pseudization procedure of the augmentation part or to
truncation at ﬁnite cutoff may give convergence problems. Raising
the ecutrho cutoff for charge density will usually help, especially in
gradient-corrected calculations."

So in short:
1.) reduce mixing_beta
2.) try different mixing_mode
3.) increase mixing_ndim (beware of the increased memory needed)
4.) add a few bands + smearing and broadening
5.) increase ecutrho

since you already tried reducing mixing_beta and adding some bands and
smearing maybe the other points do the trick for you?

Also what works for me most of the time is reduced mixing_beta in
combination with additional bands + smearing.

Hope this helps

Regards

Dominik

> 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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