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

[Fabrizio Cossu]

   Dear Julien,
what smearing did you have in mind? Have you tried raising the broadening?

   Cheers,
   Fabrizio

On Thu, 21 Feb 2019 at 12:19, Julien Barbaud <julien_barbaud at sjtu.edu.cn>
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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-- 

*Fabrizio Cossupostdoctoral fellow at APCTP (Asia Pacific Center for
Theoretical Physics)*,
Hogil Kim Memorial Building #501
POSTECH, 67 Cheongam-Ro, Nam-Gu,
Pohang-si, Gyeongsangbuk-do,
790-784 (37673), Republic of Korea

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