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[matthieu verstraete <matthieu.jean.verstraete@gmail.com>]

2 things:

1) you are probably not entirely converged: ecut needs to be higher
for almost any C pseudo I have seen, and graphene needs a _lot_ of
kpoints 12x12 may not be enough...
However, if the symmetry is correct this should not be the problem,
and you should have a degenerate state even if everything is
unconverged. Your vacuum looks sufficient (another reason for gap
opening).

2) your K point is not super precise (add more "3" digits) but
especially I am not convinced that your cell and xangst are precise
enough. If you break the perfect hexagonal lattice you will get a gap.
Check that the space group is recognized correctly and visualize
everything to check the bond lengths etc are all identical and
correct. Why don't you use reduced coordinates (0 0 0  1/3 1/3 0)and
angdeg 90 90 60 to generate the ideal cell? Your way (xcart and wierd
cell) is error prone and more complicated to use.


Matthieu

On Mon, Apr 27, 2009 at 10:40 PM,  <fishzhou@gmail.com> wrote:
> Hi there,
>
> I'm using LDA to calculate the band structure of 1-layer graphene sheet. 
> but I
> get a NON-ZERO energy gap (about 0.3eV) at kpoint "K", which doesn't agree 
> with
> other results. Could any one please tell me what's going on?
>
> Thanks a lot
>
> Aaron
>
> =======================input file=================
> ndtset 2
>
> # Dataset 1 : usual self-consistent calculation for ground state charge 
> density
> kptopt1   1
> ngkpt1    12 12 1
> nshiftk1  4
> shiftk1   0.0 0.0 0.0
>          0.5 0.5 0.0
>          0.0 0.5 0.0
>          0.5 0.0 0.0
>
> toldfe1  1.0d-7
> prtden1  1
> nband1   32
>
> ######################################################
> # Dataset 2: calculation of kss file
> kptopt2   -3
> nband2    10
> ndivk2    20 20 20
>
> kptbounds2   0.6666666666667 0.333333 0.0
>             0.0 0.0 0.0
>             0.5 0.0 0.0
>             0.6666666666667 0.333333 0.0
>
> getden2   -1
> iscf2     -2
> tolwfr2   1.0d-12
> enunit2   1
>
> # Definition of the unit cell
> acell  4.6384188886E+00  4.0166196155E+00  2.8345891875E+01
> rprim  1.0  0.0  0.0
>       0.5  1.0  0.0
>       0.0  0.0  1.0
>
> # Definition of the atom types
> ntypat 1
> znucl  6
>
> # Definition of the atoms
> natom  2              # total Number of ATOMs in the unit cell
> typat  2*1            # TYPE of atoms
> xangst
>  -1.4377960534E-21  2.0000000000E-05  0.0000000000E+00
>   1.2272700000E+00  7.0848000000E-01  0.0000000000E+00
>
> ecut 16.0         # Maximal kinetic energy cut-off, in Hartree
> nstep   200       # Maximal number of SCF cycles
> diemac  4.0       # model DIElectric MACroscopic constant
> symmorphi 0       # must 0 for GW, default 1
> timopt -1          # TIMing OPTion, -1 a full analysis of timings
>



-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Matthieu Verstraete

European Theoretical Spectroscopy Facility (ETSF)
Dpto. Fisica de Materiales,
U. del Pais Vasco,
Centro Joxe Mari Korta, Av. de Tolosa, 72,   Phone: +34-943018393
E-20018 Donostia-San Sebastian, Spain        Fax  : +34-943018390

Mail : matthieu.jean.verstraete@gmail.com
http://www-users.york.ac.uk/~mjv500