[Wien] band gap with EV-GGA and PBE-GGA

[Hong Jiang]

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 <rasofi at hotmail.com>/* 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 <us.white at yahoo.com>/
>         Reply-To:  /A Mailing list for WIEN2k users
>         <wien at zeus.theochem.tuwien.ac.at>/
>         To:  /wien2k <wien at zeus.theochem.tuwien.ac.at>/
>         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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