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

[Pietro Davide Delugas]

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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