[Pw_forum] Silicon HSE Problem

Thu May 14 13:05:06 CEST 2015

Hey,

By using a value of 0.096, (Heyd recommends using a value smaller than 0.15 to reproduce accurate band gaps in semi-conductors), my cutoff energies oscillate as follows; (scf energy, ecut energy).

-14.56060124
-14.56579225  -- 20 Ry
-14.56576890
-14.56671699  -- 30 Ry
-14.56731443
-14.56796231 -- 40 Ry
-14.56725086
-14.56720876 -- 50 Ry
-14.56751976
-14.56750677 -- 60 Ry
-14.56749110
-14.56713377 -- 70 Ry
-14.56711227
-14.56709019 -- 80 Ry
-14.56706889
-14.56704906 -- 90 Ry
-14.56722966
-14.56721482 -- 100 Ry
-14.56700415
-14.56699186 -- 110 Ry
-14.56697995
-14.56696887 -- 120 Ry
-14.56695845
-14.56694874 -- 130 Ry
-14.56693973
-14.56693137 -- 140 Ry
-14.56692360
-14.56691649 -- 150 Ry
-14.56690962
-14.56690322 -- 160 Ry
-14.56698048
-14.56697473 -- 170 Ry
-14.56696928
-14.56696413 -- 180 Ry
-14.56693442
-14.56692978 -- 190 Ry
-14.56692543

So the oscillation isn't massive, but I am only using a 2-atom cell. I am basically interested in calculating the energy of my system at elevated electronic temperatures (using femi-dirac smearing), I'm going to go ahead and calculate my band-gap and lattice parameters using this oscillating ecut, and see how close they are to literature. Do you think this is ill advised?

Thanks,

Galvin

________________________________
From: Layla Martin-Samos <lmartinsamos at gmail.com>
Sent: 14 May 2015 09:57
To: PWSCF Forum
Subject: Re: [Pw_forum] Silicon HSE Problem

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<mailto: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<mailto: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<mailto: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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