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

[Lorenzo Paulatto]

> 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 <paulatz at gmail.com 
> <mailto: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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