Dear Galvin,
Just a minor comment: How many k points did you use in this test? I seem
to remember that the electronic structure of silicon (even) in the
eight-atom cell and Gamma-point only was completely wrong, was it even
metallic (with GGA). Please do the tests with a converged set of k points.
Beyond that, I do not like the idea of using the extra parametre in the
HSE06 functional for "tuning the band gap"; of course this is not an ab
initio calculation anyway, but I would still stick to a "reasonable" value
throughout. Just my opinion...
Greetings from Montrouge,
apsi
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
Ari Paavo Seitsonen / [email protected] / http://www.iki.fi/~apsi/
Ecole Normale Supérieure (ENS), Département de Chimie, Paris
Mobile (F) : +33 789 37 24 25 (CH) : +41 79 71 90 935
On Thu, 14 May 2015, Khara, Galvin wrote:
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 <[email protected]>
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 <[email protected]>:
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 <[email protected]>
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 <[email protected]>:
> ?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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