[Pw_forum] Silicon HSE Problem

[Khara, Galvin]

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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