# [Pw_forum] Silicon HSE Problem

Khara, Galvin galvin.khara.12 at ucl.ac.uk
Fri May 22 08:11:22 CEST 2015

```Hey,

Thanks for your reply. Using an ecut of 60 Ry, a 8x8x8 kpoint grid (and also an 8x8x8 nqx grid), and a screening parameter of 0.0916, I got a value of 1.19 eV using the method you outlined. Is that a bit too good to be true? Heyd's original thesis quotes a value of 1.31 eV for silicon, but that was using a screening parameter of 0.15, which may account for the difference. Have you attempted to obtain bandstructure graph by using a defined uniform k-point list for the 2nd scf calculation? I'd like to know how to do this just with the eigenvalues (sorry for the novice questions), and thanks again for your reply.

Galvin,
University College London
________________________________
From: DHIRENDRA VAIDYA <dhirendra22121987 at gmail.com>
Sent: 20 May 2015 20:40
To: PWSCF Forum
Subject: Re: [Pw_forum] Silicon HSE Problem

Hi Galvin,

I am also looking at HSE bandstructure calculaitons, mainly for Group IV elements from semiconductore point of view. I get from PWSCF forum that its difficult to obtain the bandstructure using HSE, but bandgap might be easy to obtain. Try this,
In scf calculations add, nbnd (=8 if using 2 atom FCC lattice for Silicon). Remove 'occumpations='smearing''. And then do scf calculations first without HSE and then with HSE with 'restart_mode='restart''. In output file you will find something like this,
highest occupied, lowest unoccupied level (ev):     #some_value1    #some_value2

the difference is the bandgap = #some_value2-#some_value1

On Wed, May 20, 2015 at 6:54 PM, Khara, Galvin <galvin.khara.12 at ucl.ac.uk<mailto:galvin.khara.12 at ucl.ac.uk>> wrote:

Hey,

I'm getting a value of 5.445 Angstroms for my lattice paramter, which is pretty close to the value quoted by Heyd (5.45), and that using a different screening parameter (which he says does alter the physical properties anyway). The next thing I want to do is calculate the band gap, searching through the archives actually gives loads of topics on this very problem. However, I'm really new to DFT, is there any chance you could provide a method to do this, so far I've understood;

(i) - Do a regular scf calculation using an automatic fine k-point mesh (I'm running an 8x8x8 simulation now)

(ii) - Run another scf calculation with a defined k-point list (does this k-point list depend at all on my choice of NxNxN in (i)). Can I choose any the standard 28 k-points quoted in example01?

(iii) - How do I actually calculate my band gap from the eigenvalues outputted from this 2nd scf?

Sorry about my ignorance on this. To compare it with the literature band gap, would it also be possible to just use the dos post processing tool? I will be needing the dos at some point in the future anyway. Is this used in the same way as it is for non-hybrid functionals?

Thank you so much for taking the time to read this. It's really appreciated.

Galvin,
UCL.
________________________________
From: Layla Martin-Samos <lmartinsamos at gmail.com<mailto:lmartinsamos at gmail.com>>
Sent: 14 May 2015 13:10

To: PWSCF Forum
Subject: Re: [Pw_forum] Silicon HSE Problem

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<mailto: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<mailto: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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--
--
Dhirendra
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