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

[Lorenzo Paulatto]

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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