[Pw_forum] Silicon HSE Problem

[Ari P Seitsonen]

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 / Ari.P.Seitsonen at iki.fi / 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 <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
> 
> 
> 
>