[DFTB-Plus-User] Cohesive energy of graphene

[Bálint Aradi]

Dear Jiatao Sun,

> The cohesive energy of graphene is calculated by subtracting the energy
> of isolated carbon atom from the total energy of graphene (being 47.3358
> eV/crystal).
> The energy (being 19.0689 eV/atom) of isolated carbon atom is calculated
> by putting a single carbon atom into a (10AA x 11 AA x 12AA) unit cell
> (AA stands for angstrom unit) spin-nonrestricted.
> The obtained cohesive energy of graphene is 4.6 eV/atom. 
> This energy is considerably underestimated in comparison with VASP (a
> plain wave pseudopotential based package) being 9.06 eV/atom and
> reference J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 4907–4913 (being
> 8.96eV/atom in table 3).
> 
> I have used pbc-0-1 parameters with released
> dftb+_1.2rc1.i686-linux compiled version. Could any body give me some
> ideas where is the problem?

 Usually, I do not compare atomisation energies in DFTB as they are
somewhat ill defined. If you calculate an atom in DFTB+, you do not have
any twocenter interactions and, therefore, no corrections (no repulsive)
which should ensure that you get similar results to ab initio. It is
usually better to compare heat of formation or similar quantitites where
you compare bonded systems respectively to each other.

  Best regards,

  Bálint

-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b_aradi/


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