[QE-users] PHONON: no convergence in scf

[Fabrizio Cossu]

    Dear Quantum ESPRESSO users,
I'm trying to compute the phonon dispersion of bulk and single layer NbSe2.
While I succeed for the single layer, I fail for the bulk. I have read
previous posts about this issue and tried to reduce alpha_mix, but the
non-convergence just moves from one mode to another.
   Following the recipe from a ICTP school in March 2018
<http://epw.org.uk/School2018/School2018?action=downloadman&upname=Wed.4.Verdi.pdf>,
I'm using the inputs in attachment (up to where it fails, anyway). For the
single layer, which is successful, I have never obtained this problem (with
or without vdW corrections, even without the assume_isolated = '2D'
prescription, and with any k-mesh and q-mesh used so far).
   I tried several setups also for the bulk, with the following behaviour:

   no convergence in the bulk for k-mesh 45x45x5, q-mesh 15x15x1 or 9x9x3,
   convergence in the bulk for k-mesh 9x9x5, q-mesh 3x3x1 (only 3 points).

   I understand that a system can be far from convergence, but why does
this work for its 2D allotrope, and why does it work when I use a coarser
grid (without even adjusting the broadening, btw)? More important, what can
I do in order to obtain convergence? This system has been already
investigated in the bulk, at least in Phys Rev B 80 241108
<https://journals.aps.org/prb/abstract/10.1103/PhysRevB.80.241108> and a
more recent PRB
<https://journals.aps.org/prb/abstract/10.1103/PhysRevB.87.245111#> (btw,
isn't the broadening a bit high in those papers, or should I increase the
broadening in my case? which goes back to the question, on why the single
layer is not affected by a small broadening ...)

   Thank you in advance for any help or comment you may have about this
issue.

   Kind regards,
   Fabrizio

-- 

*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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