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, "t...@theochem.tuwien.ac.at" <t...@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_sai...@yahoo.fr <mailto:halim_sai...@yahoo.fr>>
>Reply-To: A Mailing list for WIEN2k users <wien@zeus.theochem.tuwien.ac.at <mailto:wien@zeus.theochem.tuwien.ac.at>> >To: "wien@zeus.theochem.tuwien.ac.at <mailto:wien@zeus.theochem.tuwien.ac.at>" <wien@zeus.theochem.tuwien.ac.at <mailto:wien@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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