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

[Zahra Khatibi]

Hello Lorenzo,

Thank you for the suggestions. I'm using QE 6.4 and 6.5 so the 'ppcg'
option is there among the options for diagonalization flag. I've used it as
you suggested and it workes really fast.
Also, I changed the pps from PAW to USPP out of curiosity and I can see
that it works twice faster. Any thoughts?

Wish you the best,
--
Z. Khatibi
Postdoctoral fellow
School of Physics
Trinity College Dublin


On Mon, Dec 14, 2020 at 2:11 PM Lorenzo Paulatto <paulatz at gmail.com> wrote:

> p.s. If you can use a newer version of QE that does calculation="ppcg" I
> found it to be much (i.e. 6x) faster in this case
>
> cheers
>
> On 2020-12-14 14:50, Lorenzo Paulatto wrote:
> > Hello,
> >
> > I've had a look at the output, and a part for the cutoff which appears
> > a bit too high (you are probably safe with 50/400Ry of
> > ecutwfc/ecutrho) I only see to small problems:
> >
> > 1. the scf calculation is using 6 pools with 10 k-points, which means
> > that 4 pools have twice as much work to do as the others. In the ideal
> > case, the number of pools should be a divisor of the number of
> > k-points (i.e. 2, 5 or 10 in your case). Also, it is recommended that
> > the number of CPUs in a pool are a divisor of the number of CPUs on
> > each computing node, to avoid too much inter-node communication. In
> > your case, the best choice with 72 CPUs (on two nodes?) could be 2
> > pools. You may gain a bit of time, but this is not going to change a
> > lot. You should consider using more CPUs if you have the budget. For
> > example, 10 pools of 12 or 18 CPUs each.
> >
> > 2. The bands calculation runs on 12 CPUs and has a single k-point,
> > while each pool of the SCF one has up to 2 k-points. We would expect
> > that the bands calculation take about half as an scf step, i.e. about
> > 50 seconds. However, the bands calculation has some trouble
> > diagonalizing the Hamiltonian, you see it writes:
> >
> >      ethr =  2.76E-12,  avg # of iterations =120.0
> >
> > while typically the very last scf diagonalization is
> >
> >      ethr =  2.98E-12,  avg # of iterations =  3.3
> >
> > This is because, the scf calculation can start with a very good guess
> > good the wavefunction, while the bands calculation does not. It is
> > still faster than doing the entire scf procedure, but just by a factor
> > ~2.3
> >
> > Fortunately, you do not usually need the eigenvalues to a precision of
> > 10e-12. You can set the threshold by hand using the keyword
> > diago_thr_init, I guess 1.d-6 should be tight enough. However, double
> > check what you get in output, because I am half-suspecting that it may
> > be over-written by the value in the restart file
> >
> > cheers
> >
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