[DFTB-Plus-User] Cohesive energy of graphene

[Jiatao Sun]

Dear DFTB+ users,

I have a question about the cohesive energy of single layer graphite,
namely graphene.

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?

Jiatao SUN

Research Fellow
National University of Singapore
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