[Pw_forum] Silicon HSE Problem
Layla Martin-Samos
lmartinsamos at gmail.com
Thu May 14 14:10:54 CEST 2015
good! check and see if it is within the litt. values.
cheers
Layla
2015-05-14 13:05 GMT+02:00 Khara, Galvin <galvin.khara.12 at ucl.ac.uk>:
> 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>:
>
>> 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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>
>
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