[Pw_forum] Silicon HSE Problem

[Layla Martin-Samos]

Hi, I think that the safest way is to increase the dimension of your
simulation cell (by replicating the unit cell along the three axes) and use
the screening param that gives you the correct gap. To have uncontrolled
noise may be a source of "random numbers". However, each researcher is
responsible for the numbers he publish. The impact of noise depends also on
the precision and accuracy that you will need or that you expect to need
for further post-processing/analysis/multiscale ... . Personally, I try to
have initial steps as good as I can, because for my research a
not-that-good initial step has a big and negative impact in the next steps.

cheers

Layla

2015-05-12 15:19 GMT+02:00 Khara, Galvin <galvin.khara.12 at ucl.ac.uk>:

>  Hey,
>
>
>  Sorry to bring up an old topic, but from reading the HSE paper, I need
> the screening parameter to be between 0.1 and 0.2 for a correct band gap
> calculation in silicon. Is it a bad idea to use a value for ecut that is
> oscillating (but not by a huge amount)?
>
>
>  Thanks,
>
>
>    Galvin
>     ------------------------------
> *From:* Layla Martin-Samos <lmartinsamos at gmail.com>
> *Sent:* 10 April 2015 14:39
> *To:* PWSCF Forum
> *Subject:* Re: [Pw_forum] Silicon HSE Problem
>
>   Hi Galvin, maybe the screening_parameter ( HSE has two parameters
> exx_fraction and screening_parameter) is not big/small (I never
> remember if it is dividing or multiplying) enough for your small cell.
> Maybe you could try to check the behavior of the total Energy
> convergence with respect to variations of this parameter. (in
> principle this parameter control the decay of the short range part of
> the coulomb potential in your cell. If the short range part is not
> small enough at the borders you may find funny results).
>
> cheers
>
> Layla
>
> 2015-04-10 15:20 UTC+02:00, Khara, Galvin <galvin.khara.12 at ucl.ac.uk>:
> > ?Hey,
> >
> >
> >
> > I've been trying to use the HSE hybrid in QE to do some simulations on
> Si.
> > I'm finding that increasing my ecutwfc is causing the total energy to
> > oscillate quite significantly. This seems to indicate a big problem as
> this
> > should always converge as it is variational.? When I use the plain PBE
> > (Si.pbe-hgh.upf) pseudopotential without invoking any hybrid functional
> the
> > energy converges as expected.
> >
> >
> >  It only starts to oscillate when I add the  input_dft='hse' directive, I
> > have not added anything non-standard to my input as far as I know (the
> > default values for nqx, x_gamma_extrapolation, and exxdiv_treatment? are
> > used). My input file is as follows,
> >
> >
> > "
> >
> > &control
> >     prefix='silicon',
> >     pseudo_dir = '~/pseudo'
> >     outdir='~/tmp'
> >     tprnfor = .true.
> >     tstress = .true.
> >  /
> >  &system
> >     ibrav = 1, celldm(1) =10.18, nat= 8, ntyp= 1,
> >     ecutwfc = $alat,
> >     occupations='smearing',
> >     degauss=0.002, smearing='gaussian',
> >     input_dft='hse'
> > /
> >  &electrons
> >     scf_must_converge= .true.
> >     conv_thr = 1.0e-12
> >     mixing_beta = 0.7
> >  /
> > ATOMIC_SPECIES
> >  Si 28.0855 Si.pbe-hgh.upf
> >
> > ATOMIC_POSITIONS crystal
> >  Si 0.0   0.0  0.0
> >  Si 0.5   0.5  0.0
> >  Si 0.5   0.0  0.5
> >  Si 0.0   0.5  0.5
> >  Si 0.25 0.25 0.25
> >  Si 0.75 0.75 0.25
> >  Si 0.75 0.25 0.75
> >  Si 0.25 0.75 0.75
> >
> > K_POINTS gamma
> >
> >
> > "
> >
> >
> > Also, I ultimately want to use the HSE hybrid to study Si at elevated
> > electronic temperatures (up to 25,000 K) via fermi-dirac broadening, is
> this
> > unrealistic / unwise with this choice of hybrid? Any help is greatly
> > appreciated.
> >
> >
> >
> > Thanks,
> >
> >
> > Galvin K,
> > Ph.D Researcher,
> > University College London
> >
>
>
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