[Pw_forum] very large value 'U' obtained for 'GaAs' by linear response method
Bramha Pandey
pandey.bramha at gmail.com
Mon Oct 8 15:35:10 CEST 2012
Thank You very much Dear Prof. Burak, Prof Giuseppe and Dear Prof. Matteo
for giving such a explanation for my problem. Thank you very much once
again to all my DEAR eminent Prof. for your kind attention and help.
On Mon, Oct 8, 2012 at 2:54 PM, Giuseppe Mattioli <
giuseppe.mattioli at ism.cnr.it> wrote:
>
>
> Dear all
>
> Burak has indeed described well the problem, but something more can be
> said: the Ga 3d shell (as well as the Zn 3d shell in ZnO) is almost
> atomic-like and give rise to a very narrow band in III-V
> semiconductors, horribly affected by delocalisation (aka
> self-interaction) problems. This band is raised in energy far more
> than the As (3p) valence band; so the two bands are mixed (and they
> should not) and the valence band maximum is pushed up towards the
> conduction band minimum. Moreover, you cannot extract meaningful
> self-consistent U values by linear response procedures in the limit of
> a simplified "on-site, one-shell" approach. However, it is not
> forbidden to use "semi-empirical" U values, as long as you have a good
> knowledge of the physics of your system... The above problems in DFT
> calculations of GaAs and ZnO are addressed in a Zunger paper, very
> long and quite... difficult to flick through.
>
> Lany, S. and Zunger, A.; Phys. Rev. B 78, 235104 ͑(2008͒).
>
> In a short, you may try to decouple the Ga (3d) band from the As (3p)
> band by hammering with some empirical U value the head of the Ga 3d
> band. And you can recover a very good band gap value by pushing down
> (more softly...) the As 3p band. You may test your results against a
> hybrid functional. I've done it in the case of ZnO, and I've obtained
> quite good results.
>
> Matteo suggest that the Ga 3d has to be included in the calculation.
> In my experience (but I'm the "empirical" kind of computational
> guy...) The Ga 3d shell is mandatory in the case of GaN if you want a
> good lattice parameter. In the case of GaAs you can obtain good
> results (band gap included) by embedding the Ga 3d electrons within
> the Ga pseudopotential, and by using a NLCC approach.
> This leads to some kind of decoupling of the Ga 3d and As 3p bands. I
> remember some old papers (2000-2005) by Van de Walle and Neugebauer on
> GaN in which a Ga PP without 3d shell was used after a lattice
> parameter optimisation performed by using a Ga PP with 3d shell.
>
> HTH
>
> Giuseppe
>
> Quoting Burak Himmetoglu <himm0013 at umn.edu>:
>
> > Dear Brahma,
> >
> > Ga has a full d-shell. Therefore, if you try to perturb the d-shell of
> Ga,
> > you can reduce the number of electrons localized on d-states by a
> positive
> > Hubbard_alpha but there is no way to increase their occupations with a
> > negative Hubbard_alpha. Therefore, the response matrices will be
> non-linear
> > and you will get very small numbers in them. When you invert these
> > matrices, very large numbers will appear.
> >
> > Calculating U for full d-shells is not easy, and there is no simple
> > solution (Another known example is ZnO). My personal opinion is that the
> > use of LDA+U is questionable for filled d-shells. In fact if you check
> the
> > paper mentioned by Matteo, the gap for GaAs is corrected by applying U
> and
> > V (inter-site interactions) on Ga-p states and states which overlap with
> > Ga-p. No correction on Ga-d is applied.
> >
> > Best regards,
> >
> > Burak
> >
> > On Sun, Oct 7, 2012 at 12:02 PM, Bramha Pandey
> > <pandey.bramha at gmail.com>wrote:
> >
> >> sorry for wrong matrix copied in previous mail.
> >>
> >> CHI0^-1 - CHI^-1 Matrix
> >>
> >> 0.0000 0.0000 0.0000
> >> 0.0000 0.0000 0.0000
> >>
> >> 0.0000 0.0000 0.0000
> >> type: 1 U0 = 0.000000000000000E+000
> >>
> >> On Sun, Oct 7, 2012 at 10:30 PM, Bramha Pandey
> >> <pandey.bramha at gmail.com>wrote:
> >>
> >>> Dear Prof. Matteo,
> >>> i am using Ga, pseudo with 3d as a valence, Hubbard_l=2 is set for
> 'Ga'.
> >>> If it should change how much value i have to set it?
> >>>
> >>> Dear sir i am totally struck by zero matrix element in ch0^-1 - chi^-1,
> >>> so always it gifted me ZERO value where as ch0 and chi matrix are NON
> ZERO
> >>> with same element value.
> >>> CHI_0 Matrix
> >>> -0.0002 -0.0002 0.0005
> >>> -0.0002 -0.0002 0.0005
> >>> 0.0005 0.0005 -0.0010
> >>> CHI Matrix
> >>> -0.0002 -0.0002 0.0005
> >>> -0.0002 -0.0002 0.0005
> >>> 0.0005 0.0005 -0.0010
> >>>
> >>> CHI0^-1 - CHI^-1 Matrix
> >>>
> >>> 0.0000 0.0000
> >>> 0.0000 0.0000
> >>> type: 1 U0 = 0.000000000000000E+000
> >>> This above is for all the 2 atoms supercell combination when we take
> the
> >>> ZB structure with 2 atoms: (1) when both atoms are 'Ga'
> >>> (2) when both atoms are 'Ga1' and 'Ga' (3) when both atoms are
> different
> >>> 'Ga' and 'As'.
> >>>
> >>> All above three conditions are tested under one atom(Ga1 or Ga) is
> >>> perturbed.
> >>> Please Sir give me some ray of hope so that i can get rid of this
> >>> situation.
> >>>
> >>>
> >>> On Sun, Oct 7, 2012 at 8:10 PM, Matteo Cococcioni <matteo at umn.edu>
> wrote:
> >>>
> >>>> Dear Bramha,
> >>>>
> >>>> in J. Phys.: Condens. Matter 22 (2010) 055602 we did a similar
> >>>> calculation but we got pretty different numbers (see Table 2 of the
> >>>> reference). Maybe the states of Ga you applied U or alpha to are
> >>>> completely full. what is the value of l set for Ga in set_hubbard_l ?
> >>>> did you change that? also, for Ga, you better use a pseudo with 3d
> >>>> states in the valence rather than frozen in the core.
> >>>>
> >>>> Matteo
> >>>>
> >>>>
> >>>>
> >>>> On Sun, Oct 7, 2012 at 7:31 AM, Bramha Pandey <
> pandey.bramha at gmail.com>
> >>>> wrote:
> >>>> > First of all thank you very much Dear Prof.Stefano.
> >>>> > I have change the input as ZB 'GaAs' structure and calculate the 'U'
> >>>> value
> >>>> > for both 'Ga' and 'As' which is given bellow.
> >>>> > But i wondered to see the 'U' value of 'Ga' 1010.79 which is quite
> >>>> large so
> >>>> > how can i figure out this is correct or wrong?
> >>>> > No of Atom U_Ga U_As
> >>>> > 2 1043.35792360332 2.87817590827985
> >>>> > 16 1010.81003743078 2.82970777976411
> >>>> > 54 1010.79529573193 2.82968584264055
> >>>> > 128 1010.79528904780 2.82968583269392
> >>>> > 250 1010.79528904476 2.82968583268940
> >>>> > Any comment is welcome.
> >>>> > --
> >>>> > Thanks and Regards
> >>>> > Bramha Prasad Pandey
> >>>> > Ph.D Student Indian School of Mines(ISM)
> >>>> > Dhanbad, INDIA.
> >>>> >
> >>>> >
> >>>> > _______________________________________________
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> >>>>
> >>>>
> >>>>
> >>>> --
> >>>> Matteo Cococcioni
> >>>> Department of Chemical Engineering and Materials Science,
> >>>> University of Minnesota
> >>>> 421 Washington Av. SE
> >>>> Minneapolis, MN 55455
> >>>> Tel. +1 612 624 9056 Fax +1 612 626 7246
> >>>> _______________________________________________
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> >>>>
> >>>
> >>>
> >>>
> >>> --
> >>> Thanks and Regards
> >>> Bramha Prasad Pandey
> >>> Ph.D Student Indian School of Mines(ISM)
> >>> Dhanbad, INDIA.
> >>>
> >>>
> >>
> >>
> >> --
> >> Thanks and Regards
> >> Bramha Prasad Pandey
> >> Ph.D Student Indian School of Mines(ISM)
> >> Dhanbad, INDIA.
> >>
> >>
> >> _______________________________________________
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> >>
> >
>
> Giuseppe Mattioli
> ISM-CNR
> Italy
>
>
>
>
>
>
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--
Thanks and Regards
Bramha Prasad Pandey
Ph.D Student Indian School of Mines(ISM)
Dhanbad, INDIA.
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