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

[Mary White]

Dear Khuong,
     Thank you for your quick reply.
     I optimized the structure with PBE-GGA. The lattice parameters of  the experimental structure are 8.283a.u. and18.939a.u. While the optimized parameters are 8.387a.u. and 19.172a.u. The difference is not very much.But the gap is 0.2ev compared with 0.1ev with experimental structure.

"Khuong P. Ong" <ongpk at ihpc.a-star.edu.sg> wrote:   Hi,

  Did you optimize structure by using EV-GGA? If Yes, it is NOT correct. Please notice that never use EV-GGA for optimization. It will give you wrong results. If you use PBE-GGA, how much is the difference between experimental and theoretical result for lattice parameters?

 Regards,
 Khuong

 At 01:20 PM 5/24/2007, you 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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 Dr Ong Phuong Khuong 
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