[Pw_forum] How to set Occupation to Calculate Grapgene DOS?

[Lorenzo Paulatto]

On 01/23/2014 08:09 PM, Li, Run wrote:
> I’m trying to calculate the DOS of graphene, but no matter what
> functional and pseudopotential I use, I keep getting DOS=0 about 1.4eV
> around Fermi level , even though the bands calculation can get the Dirac
> point at K point, showing the conduction band is touching the valence
> bands, so graphene is a semi-metal.  I used occupation=tetrahedra in the
> nscf calculation to calculate DOS, while occupation=smearing to
> calculate bands. Should I use occupation= smearing instead? How do I get
> a reasonable DOS? (I used a super-cell with one layer of graphene in the
> bottom, and enough vacuum. I hope that’s not a problem)


Dear Run,
I'm not sure that tetrahedra give a good result in 2D systems, I would 
try gaussian smearing for the DOS too and see if it makes a difference.

On the other hand, what really matters is not the kind of smearing but 
its amount. To refine the DOS you'll have to use a smaller smearing and 
increase the number of k-points. In your case, I have the impression 
that smearing is small enough but you lack k-point. I cannot say more 
because you forgot to include the input file. You can do a NSCF 
calculation with a dense grid before the DOS calculation.

Finally (I'm not 100% sure about this), the DOS calculation will reuse 
the k-points that where defined in the previous SCF or NSCF calculation. 
Hence, doing a band calculation and then the computing the DOS is wrong: 
the DOS will be computed with the k-points used for the bands.

kind regards

-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
+33 (0)1 44 275 084 / skype: paulatz
http://www-int.impmc.upmc.fr/~paulatto/
23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05

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