[Pw_forum] Silicon HSE Problem
DHIRENDRA VAIDYA
dhirendra22121987 at gmail.com
Wed May 20 21:40:12 CEST 2015
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>
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>
> *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>:
>
>> 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
>>> >
>>>
>>>
>>> _______________________________________________
>>> Pw_forum mailing list
>>> Pw_forum at pwscf.org
>>> http://pwscf.org/mailman/listinfo/pw_forum
>>>
>>
>>
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--
--
Dhirendra
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