[Wien] band gap with EV-GGA and PBE-GGA
Hong Jiang
jiang at fhi-berlin.mpg.de
Thu May 24 16:11:04 CEST 2007
Hi Mary,
You can try using LDA+U, (orb in Wien2k) for the systems you are
interested in. (see wien2k userguide 4.5.6)
Good luck,
Hong
Mary White wrote:
> Dear all,
> Thank you for all your help. I once considered the GW
> approximation.But what I calculate is heavy atoms,I can't deal with
> heavy atoms with the GW method.Maybe in the pseudo potential,can add
> the spin-orbit coupling.Is there indeed no other ways to calculate the
> small gap in wien2k?
> Thank you for your attention.
> Mary
>
> */Hong Jiang <jiang at fhi-berlin.mpg.de>/* wrote:
>
> Hi,
> Your problem is actually not related to which structure you use.
> Usually the band gap calculated from LDA or GGA can not be taken too
> seriously because of the well-known band gap problem of LDA/GGA, i.e.
> the LDA/GGA band gaps are always under-estimated, and therefore for
> small gap systems, LDA/GGA usually predict metallic behavior; for
> transition metal oxides like MnO, FeO, CoO and NiO, even experimental
> gaps are not small (~4 eV), LDA/GGA can also give zero or very small
> gaps. In terms of the band gap, EV93 is known to give larger gaps,
> but
> usually not large enough compared to experiment. For accurate and
> reliable calculations of band gaps, one has to use the GW
> approximation, which is unfortunately very expensive and only
> feasible
> for small systems. See the review paper in Rep. Prog. Phys. 61, 237
> (1998) for more details.
> Hope this is helpful.
>
> Hong
>
>
>
>
>
> Mary White wrote:
> > Dear R.Ahmed and all users,
> > Thank you for your attention. After I first optimized the
> > experimental structure (both lattice parameters and atom
> position), I
> > calculated the band structure and got the gap of 0.2ev with EV-GGA
> > (0.1ev with the experimental structure EV-GGA). The optimized
> lattice
> > parameters is a little overestimated compared the experimental
> lattice
> > parameters. But I found the force on the two atoms is a little
> large.
> > So I optimized the atom position in min_lapw (not for lattice
> > parameters).The force is small. Then I calculate the band gap
> with the
> > new optimized structure, but very strange,the gap is disappear.
> That
> > appears metallic. I don't know what I can do. Only optimized the
> atom
> > position,the result is very different (compared with the first
> > optimized structure 0.2ev).I think the optimized structure is ok.
> > There is no experimental band structure. I don't know how to
> > deal with it. I tried to adjust the parameters,but that is no
> effect.
> > Any comments do help me.Thanks in advance.
> >
> > */Rashid Ahmed /* wrote:
> >
> > Dear Mary
> > hi
> > This is known fact that GGA usually overestimates lattice
> > parameters which susequently decrease band gap value. As your
> > result with experimental lattice parameters is already very small.
> > so with further decrease in band gap, it shows wrong mettalic band
> > structure.
> > bye
> > R. Ahmed
> >
> >
> ------------------------------------------------------------------------
> > From: /Mary White /
> > Reply-To: /A Mailing list for WIEN2k users
> > /
> > To: /wien2k /
> > Subject: /[Wien] band gap with EV-GGA and PBE-GGA/
> > Date: /Tue, 22 May 2007 05:10:18 -0700 (PDT)/
> >
> > Dear all users,
> >
> > I calculated the energy band with EV-GGA and PBE-GGA.
> > The system is semiconductor.And the gap I calculated with the
> > experimental structure is about 0.1ev with EV-GGA.But when I
> > calculate with the optimized structure,the gap is
> > disappear.From the band structure,it appears
> > metallic.The band from EV-GGA and PBE-GGA seems very
> > similar.The parameters in the calculation with experimental
> > structure is the same as that with optimized structure.I think
> > the optimized structure is OK. I am puzzled with that.
> >
> > Does anyone know it? Look forward to your advice. Any
> > help will be appreciated.Thank you in advance.
> >
> >
> >
> > Mary White
> >
> >
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