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

[Fabrizio Cossu]

   Dear Julien,
I'd go for the suggestion proposed by Pietro. Mind also that, for
semiconductor, Methfessel-Paxton is not appropriate, as far as I know. Keep
Gaussian or try Fermi smearing. What's the recommended value for the cutoff
for each pseudopotential? An insufficient cutoff may considerably affect
the results, especially for insulators (forgive me for having overlooked
the electronic character of the compound previously). Moreover, the
theoretical lattice constant may be substantially different from the
experimental one; a very different lattice constant more likely gives rise
to numerical instabilities.

   Best wishes,
   Fabrizio

On Fri, 22 Feb 2019 at 15:50, Julien Barbaud <julien_barbaud at sjtu.edu.cn>
wrote:

> Dear Fabrizio,
>
> Thank you for your very quick answer and your suggestions. To answer your
> question, I did not inlcude smearing in my most general attempts (the "by
> default" input file), because it was my understanding that semiconductors
> *should* not require it. Indeed, the charge density already decays
> smoothly to 0 around the band gap, so it should not pose the numerical
> problems that metals do. Am I mistaken ?
>
>
> However, as mentionned, I did experiment with a fix where I include some
> additional empty bands to see if that solves the problem (as suggested in:
> https://www.quantum-espresso.org/Doc/pw_user_guide/node21.html section
> 5.0.0.21). In this case, I used a Methfessel-Paxton smearing (m-p). I
> tried 30 bands (instead of 25 by default) with several values of
> broadening, but this does not appear to solve the problem.
>
> As shown in the figure attached (reporting the evolution of estimated
> accuracy at each scf iteration), there is no obvious advantage in raising
> the value of degauss. The curve still stalls after a cetain number of
> iterations.
>
> I also attached the input file used for this test.
>
>
> Julien
>
>
> Le 21/02/2019 à 12:53, Fabrizio Cossu a écrit :
>
>    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 !
>> _______________________________________________
>> users mailing list
>> users at lists.quantum-espresso.org
>> https://lists.quantum-espresso.org/mailman/listinfo/users
>
>
>
> --
>
> *Fabrizio Cossu postdoctoral 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
>
>                        |
>   .----. .----. .----. |----  .----.   ===========================
>   ,----| |    | |      |      |    |   http://www.apctp.org/?JrgId=16
>   `----^ |----' `----' `----' |----'   ===========================
>          |                    |
>
>
> _______________________________________________
> users mailing listusers at lists.quantum-espresso.orghttps://lists.quantum-espresso.org/mailman/listinfo/users
>
> _______________________________________________
> users mailing list
> users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20190222/e6e225a3/attachment.html>