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

[Michal Krompiec]

Dear Lorenzo,
Speaking of ppcg, is there any published (or otherwise public) benchmark of
ppcg vs Davidson and/or cg? For which cases can ppcg be expected to be
faster?
Best regards,
Michal Krompiec, Merck KGaA

On Mon, Dec 14, 2020 at 2:12 PM Lorenzo Paulatto <paulatz at gmail.com> wrote:

> p.p.s I mean diagonalization='ppcg'
>
> On 2020-12-14 15:10, Lorenzo Paulatto 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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