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.
For comparison, I have here a calculation with 119 electrons, 10
k-points, 100 Ry kinetic energy cutoff. One SCF iteration takes about 5
seconds on 32 CPUs (2 nodes of a very old computing cluster that has
since been retired). From 120 to 190 electrons there should be around a
factor 4 of CPU times. But it would be easier to say which is the source
of the discrepancy if you sent your input and output files to teh list,
to have a look
cheers
All the best,
Zahra
On Fri, Dec 11, 2020, 22:22 Lorenzo Paulatto <[email protected]
<mailto:[email protected]>> 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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