[Wien] Why Wien2k does not provide proper energy degeneracy for graphene?

[Peter Blaha]

Check <our atomic positions. The C positions of 1/3 and 2/3 should be 
"correct" to max. number of digits (+ rounding), but most likely they 
cannot be specified with "sufficient" precision to obtain "better" 
degeneracy.

Martin Gmitra schrieb:
> Dear prof. Blaha,
> 
> We have observed using Wien2k_08.3 an artificial energy difference for free
> standing graphene at the K-point, where bands have to cross. The 
> degenerated
> energies at the K-point are separated of about 4.15 micro eV when full
> symmetry (NSYM=24) of the system is considered. Interestingly, when
> 2 inequivalent C atoms are considered without inversion symmetry (NSYM=6)
> the energies at the K-point differ of about 0.28 micro eV only. For both the
> cases the Rkmax=7, 33x33 k-mesh and Gmax=20 is considered. Why are
> the results more accurate in the less symmetric case?
> 
> I am giving here more hints from my test runs. The following does not 
> reduce
> the energy difference:
> 1. increasing k-mesh
> 2. increasing Rkmax
> 3. increasing number of LMs
> 4. increasing IFFT parameter
> 
> With many thanks in advance,
> Martin Gmitra
> 
> p.s.: I am attaching a figure of the energy dispersions
> 
> 
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-- 
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Peter Blaha
Inst. Materials Chemistry, TU Vienna
Getreidemarkt 9, A-1060 Vienna, Austria
Tel: +43-1-5880115671
Fax: +43-1-5880115698
email: pblaha at theochem.tuwien.ac.at
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