[Pw_forum] testing HSE for BCC Fe

Fri Sep 9 11:50:35 CEST 2016

Dear Daniel, Stefano and Paolo.

My added penny: in my hands US/EXX has never worked in a useful way...:-( Moreover there is a nasty constraint that force ecutrho=4*ecutwfc, that is 
not exactly the best for US PPs.
It might turn out that a NC/EXX calculation (always very satisfying in my hands) is significantly faster end quite reliable, even if you must use 
80/320 Ry cutoffs (or even higher; first row transition metals with 2s,2p semicore electrons are usually the hardest NC PPs).

You can try to speedup the calculation by using

a) adaptive_thr=.true.

In the case of smooth convergence this makes inner scf cycles significantly faster.

b) ecutfock~[1/2-->1/4]*ecutrho

Fantastic with isolated systems; handle with care with metals because this might change occupations at k-points. You should check convergence on this, 
but in the case of "almost metallic" systems (e.g., shallow defects in semiconductors) ecutfock=1/2*ecutrho is often well converged.

c) remember that (at least in principle) you should check convergence with respect to the projection of density in real space: you are (implicitly) 
using one q-point for each k-point, having not assigned any value to nqx1, nqx2, nqx3. More q points increase the computational effort. You might try 
to reduce the k grid and to increase nqx(i) values to find a correct balance: However a 7 7 7 0 0 0 grid is actually large. You will not be able to 
perform 'nscf' or 'bands' calculations to have a proper band structure, and densities/forces/stresses/eigenvalues/energies should converge at an 
acceptable level with less k points.

HTH
Giuseppe

On Friday, September 09, 2016 10:42:49 AM Daniel Stoeffler wrote:
> Hello Stefano,
> 
> As said, I am a new user of QE but not of the DFT
> ;). I use the FLEUR, Abinit and the VASP codes and I want to test if it
> would be interesting for my studies to use also QE.
> 
> I said
> "surprisingly" huge because I compared to the VASP code with the same
> parameters (Ecutoff, NKpoints, Nq) and with the HSE functional. Indeed,
> with the VASP code during one night on my computer I got a few tens of
> iterations compared to one with QE :(.
> 
> >From my point of view, I
> 
> compare 2 pseudopotential-based methods for the same parameters which I
> can control via the input file : my question would then more preferably
> be "which default parameter should I modify to make the QE calculation
> time similar to the VASP one ?"
> 
> Best wishes,
> 
> Daniel
> 
> Le 2016-09-09
> 
> 09:47, Stefano de Gironcoli a écrit :
> > Please read the literature
> 
> about hybrid functionals in plane waves; understand why the
> computational time is, not unexpectedly, huge and start think about it.
> 
> > If you come up with some smart idea on how to reduce the
> 
> computational time without compromising accuracy (as for the ACE
> transformation recently proposed by Lin Lin and implemented in the
> latest QE version) let us know
> 
> > Best,
> > stefano
> > (sent from my phone)
> > 
> > 
> > On 09 Sep 2016, at 09:30, Daniel Stoeffler
> 
> <daniel.stoeffler at ipcms.unistra.fr> wrote:
> >> Hello,
> >> 
> >> As a new
> 
> QE user, I do some tests with the HSE Functional for BCC ferromagnetic
> Fe with a cell containing 2 atoms (because a want to go to CoFe after
> that).
> 
> >> My input file is :
> >> 
> >> &control
> >> title='Fe CC
> 
> HSE',
> 
> >> calculation = 'scf',
> >> prefix = 'Fe_CC_HSE',
> 
> pseudo_dir='./',
> 
> >> verbosity = 'high'
> >> /
> >> &system
> >> ibrav = 6,
> 
> celldm(1) = 5.45,
> 
> >> celldm(3) = 1.0,
> >> nat = 2,
> >> ntyp = 1,
> 
> nspin = 2,
> 
> >> occupations='smearing', smearing='gauss', degauss=0.01,
> 
> starting_magnetization(1) = 1.0,
> 
> >> ecutwfc = 33,
> >> input_dft='HSE',
> >> 
> >> report = 5
> >> /
> >> &electrons
> >> mixing_beta = 0.7
> >> /
> 
> ATOMIC_SPECIES
> 
> >> Fe 55.845 Fe.pbe-sp-van.UPF
> >> 
> >> ATOMIC_POSITIONS
> 
> (crystal)
> 
> >> Fe 0.000 0.000 0.000
> >> Fe 0.500 0.500 0.500
> 
> K_POINTS (automatic)
> 
> >> 7 7 7 0 0 0
> >> 
> >> Surprisingly, the computation
> 
> time is huge as writen into the output file :
> 
> 
> ===============================
> 
> >> iteration # 9 ecut= 33.00 Ry
> 
> beta=0.70
> 
> >> Davidson diagonalization with overlap
> >> ethr = 5.49E-08,
> 
> avg # of iterations = 2.3
> 
> >> Magnetic moment per site:
> >> atom: 1
> 
> charge: 14.4350 magn: 2.6346 constr: 0.0000
> 
> >> atom: 2 charge: 14.4351
> 
> magn: 2.6343 constr: 0.0000
> 
> >> total cpu time spent up to now is 7.2
> 
> secs
> 
> >> End of self-consistent calculation
> >> 
> >> convergence has
> 
> been achieved in 9 iterations
> 
> >> EXX: now go back to refine exchange
> 
> calculation
> 
> >> total cpu time spent up to now is 4239.4 secs
> 
> Self-consistent Calculation
> 
> >> iteration # 1 ecut= 33.00 Ry
> 
> beta=0.70
> 
> >> Davidson diagonalization with overlap
> >> ethr = 5.49E-08,
> 
> avg # of iterations = 4.9
> 
> >> total cpu time spent up to now is
> 
> 52490.2 secs
> 
> >> iteration # 2 ecut= 33.00 Ry beta=0.70
> >> Davidson
> 
> diagonalization with overlap
> 
> >> ===============================
> >> 
> >> 
> >> So, the calculation takes 7 seconds for converging with PBE, 1h10min
> 
> for calculating the EXX term and 14h35min for the first HSE iteration.
> Is this correct ? And if so, please, could you try to explain me how to
> reduce the computation time. Thanks in advance.
> 
> >> Best wishes,
> >> 
> >> 
> >> Daniel
> >> 
> >> _______________________________________________
> 
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> 
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> 
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> 
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