[Wien] problem of energy gap within mBJ

[Gavin Abo]

In [1], it says:

/In Refs. [77,82] we showed that a correct description of the band gap 
and electric-field gradient (EFG) in Cu2O could only be achieved with 
the hybrid functionals, while the results////obtained with the LDA, GGA, 
LDA+U, and mBJ methods////were qualitatively wrong./

In TABLE IV of [2], GGA (PBE) gap is 0.53 eV.  In TABLE I of [3], mBJ 
gap is 0.82 eV.

I'm not seeing a WIEN2k value reported for GGA+U, but maybe it is close 
to the 0.79 eV gap (AMF, U = 8 eV) LDA+U calculation [2] or the 0.67 eV 
gap (with U of 7 eV) GGA+U calculation by VASP [4].

The 0.53 eV, 0.82 eV, and 0.67 eV above round up to be approximately 1.0 
eV, which is around the 1.0 eV that you report.

On the other hand, GGA+U+mBJ calculation may give 1.60 eV [6].

[1] https://publik.tuwien.ac.at/files/PubDat_238321.pdf

[2] https://publik.tuwien.ac.at/files/PubDat_197400.pdf

[3] https://publik.tuwien.ac.at/files/PubDat_197196.pdf

[4] http://dx.doi.org/10.1063/1.3231869

[5] http://dx.doi.org/10.1063/1.4798706

[6] http://dx.doi.org/10.1063/1.4798706 (TABLE I)

On 8/18/2017 9:14 PM, halim said wrote:
> Dear Prof. Tran,
>
> Hi,
>
>
> Thank you for your answer, I mean withing GGA, mBJ-GGA, and GGA+U, I 
> found the energy gap around 1 eV.
>
> In principle the experimental gap is about 2 eV.
>
> Where is the problem?
>
> Thank you for your help.
>
> Yours sincerely,
>
> Halim Said
>
> I a
>
>
> Le Vendredi 18 août 2017 23h29, "tran at theochem.tuwien.ac.at" 
> <tran at theochem.tuwien.ac.at> a écrit :
>
>
> Hi,
>
> What do you mean by 1 eV? Does it mean that all
> gaps are at exactly 1.0 eV or in a range like 0.5-1.5 eV?
>
> You can compare your results with those in these two articles:
> https://publik.tuwien.ac.at/files/PubDat_197400.pdf
> https://publik.tuwien.ac.at/files/PubDat_197196.pdf
>
> Cu2O is a special case for which mBJ and GGA+U are not really
> better than GGA.
>
> FT
>
> On Friday 2017-08-18 21:42, halim said wrote:
>
> >Date: Fri, 18 Aug 2017 21:42:24
> >From: halim said <halim_said04 at yahoo.fr <mailto:halim_said04 at yahoo.fr>>
> >Reply-To: A Mailing list for WIEN2k users 
> <wien at zeus.theochem.tuwien.ac.at <mailto:wien at zeus.theochem.tuwien.ac.at>>
> >To: "wien at zeus.theochem.tuwien.ac.at 
> <mailto:wien at zeus.theochem.tuwien.ac.at>" 
> <wien at zeus.theochem.tuwien.ac.at <mailto:wien at zeus.theochem.tuwien.ac.at>>
> >Subject: [Wien] problem of energy gap within mBJ
>
> >
> >Dear Professor Peter Blaha, Tran and Wien2k users,
> >
> >I have computed the energy gap for Cu2O by using GGA, mBJ, and GGA+U, 
> I got same energy gap of 1 eV.
> >In principle by using mBJ, the energy gap should be increased close 
> to expermimental value around 2 eV.
> >Why mBJ is giving same value as GGA and it is not improved? even I 
> tried GGA+U and the same problem and no thing was changed for all 
> approximations.
> >
> >I would be thankful for your suggestion and answer to solve the problem.
> >
> >Sincerely yours,
> >
> >Halim Said
>
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