[QE-users] time consuming band structure calculation for a supercell

[Zahra Khatibi]

Hello Lorenzo,

It's really nice to hear from you. I hope you're doing well.

So I am using QE 6.4 and unfortunately I can see that the scf calculation
takes same amount of time as the band calculation. So I doubt that the
problem might be the inconsistency between these two calculations. But
sure, I'll increase the ecutwfc. Thank you for the tip.

Aslo I have tried running the band calculation on different systems (local
pc with 12 nodes) and HPC (with 36 and 72 nodes). Every time I have the
same problem. I have tried QE 6.5 and 6.4 for this calculation all with
same issue.

All the best,
Zahra




On Fri, Dec 11, 2020, 22:22 Lorenzo Paulatto <paulatz at gmail.com> wrote:

> Hello Zahra,
>
> if I understand correctly, you manage to do the scf calculation, but then
> the band calculation is very slow. The cost per k-point of nscf should be
> more or less the same as the cost per k-point of one scf iteration. If it
> is not, there is something wrong. One possible problem, is that ecutwfc is
> interpreted differently during nscf. A tight value (1.d-12 or less) may
> cause the threshold of diagonalization in nscf to become too small and very
> slow to converge. This should be fixed in v 6.7, but you can just increase
> ecutwfc in nscf if you're using a previous version.
>
> If not, it may be a problem with parallelism, i.e. running on too many
> CPUs or some proper human error like running with all the processes on the
> same computing node.
>
>
> cheers
> On 2020-12-11 19:25, Zahra Khatibi wrote:
>
> Dear all,
>
> First of all, I hope everyone is safe and well in these crazy times.
> I'm calculating the electronic band dispersion of a 2D heterostructure
> with a 59 atom unit cell. This system is a small bandgap (10-20 meV)
> semiconductor. The number of valence bands is (valence electrons/2) 181.
> When I set 'nbnd' to 190, the band structure calculation costs me 30
> minutes for each k point on HPC with 72 processors. This means that if I do
> a simple band calculation for a high symmetry path with 100 points within,
> I have to wait almost 50 hours! This even becomes worst when I try to
> evaluate the band dispersion with SOC switched on (twice the spin
> degenerate band calculation).
> Since the band dispersion evaluation is the major part of our study, I was
> wondering if there is a way around this problem, like reducing the number
> of bands by only looking at energy interval close to Fermi energy?
> I could see that there are lots of papers and studies in the literature
> with huge unit cells and heavy atoms that have presented numerous band
> structures (using QE). So I really appreciate it if you could help me here.
>
> Kind regards,
> --
> Z. Khatibi
> School of Physics
> Trinity College Dublin
>
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