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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