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Dear users,
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<p>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.<br>
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<p> 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: <br>
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<ul>
<li> 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)</li>
<li> varying cutoff energy for charge (cf ecutrho.png)</li>
<li>taking larger k-point sampling (not shown)</li>
<li> 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)</li>
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<p>The only change that I found effective was to reduce the
mixing_beta factor.<br>
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<p>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.</p>
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<p>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.<br>
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<p>Any insight on what I did wrong would be really helpful. I
suspect a shameful beginner mistake, but can not find it out.<br>
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<p>Thanks in advance,</p>
<p>Julien barbaud</p>
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<p>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 !<br>
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