[Pw_forum] A little energy gap (~ 0.2 eV) for graphene

[Gabriele Sclauzero]

Dear Cao,

    I'm pretty sure that at the DFT level (without SO) the DOS of graphene has to be linear close to the Dirac point (which corresponds to the Fermi energy for pristine graphene).
You can easily understand this by looking at the band structure studied with the primitive unit cell. The gap that you see is due to the discrete k-point sampling and the peculiar band structure of graphene. I also think that the effect is amplified by the fact that you use the tetrahedron method, which was devised for three-dimensional systems (though it seems to work fine also for graphene). Most probably, for your k-point mesh the two closest eigenvalues across the Dirac point are spaced by about 0.2eV.
I don't think this is a real problem for your application.

HTH

GS


Il giorno 21/mag/2012, alle ore 14.13, Cao TF ha scritto:
> Dear QE users, 
> I have calculated the density of states (DOS) of pure graphene. The supercell used in my work is a rectangular cell containing 60 carbon atoms. But something has puzzled me greatly. There is always a little energy gap (~0.2 eV) in the DOS, which contracts with the fact that graphene is zero gap semiconductor. Although I have change many parameters. This problem has not been solved. Here I give the input file and I will be greatly appreciated if you can give me some suggestions. 
> For nscf.in 
> &CONTROL 
> title = 'graphene layer' , 
> calculation = 'nscf' , 
> restart_mode = 'from_scratch' , 
> outdir = './tmp' , 
> pseudo_dir = '/lustre/ISSP2/tfcao/pseudo' , 
> prefix = 'graphene' , 
> / 
> &SYSTEM 
> ibrav = 8 , 
> a = 12.31 , b = 12.79312342 , c = 12.000 , 
> nat = 60 , 
> ntyp = 1 , 
> occupations = 'tetrahedra' , 
> tot_charge = 0.0, 
> nspin = 2 , 
> starting_magnetization(1) = 0.5 , 
> ecutwfc = 30.0 , 
> ecutrho = 300.0 , 
> / 
> &ELECTRONS 
> conv_thr = 1.0d-8 , 
> mixing_mode = 'local-TF' , 
> mixing_beta = 0.7 , 
> diagonalization = 'david' , 
> / 


§ Gabriele Sclauzero, EPFL SB ITP CSEA
   PH H2 462, Station 3, CH-1015 Lausanne







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