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[Anglade Pierre-Matthieu <anglade@gmail.com>]

Hi,
 how much is your calculation converged ?
Since all the higher conduction bands have no effect on energy and you
convergence criterion (toldfe) is energy based, could it be possible
that your calculation is not converged enough on the band structure.
In case you have any reason to suspect I'm right, I suggest trying the
same calculation with a very low value for tolwfr.

regards

PMA

On Tue, Aug 18, 2009 at 11:53 PM, Aaron Zhou<fishzhou@gmail.com> wrote:
> Dear all,
>
> I have a question when calculating graphene band structure. I leave a
> 10 angstrom vacuum in Z-direction. But I saw many extra bands which
> don't exist when doing the same thing in VASP. So I did a series of
> calculation only changing the vacuum size, from 3A to 15A. I saw some
> excited bands are pushed down from higher energy. Even 8 angstrom
> vacuum and 10 angstrom vacuum will make big difference. That is quite
> unexpected! I think the vacuum is already big enough to see the
> convergence in band structure.
>
> Could someone tell me why those conduction bands are pushed down when
> changing the vacuum size (acell(3) in input file)?
>
> (The attached ppt slides will show how the vacuum affects the band 
> structure)
>
> Thanks a lot!
>
> Aaron
>
> I have at
>
> ========== INPUT FILE ============
> #one layer graphene sheet
> 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    # Will stop when this tolerance is achieved on total 
> energy
> prtden1  1         # Print out density
> nband1   32
>
>
> kptopt2   -3
> nband2    16
> ndivk2    50 50 50
>
> kptbounds2   2/3 1/3 0.0
>             0.0 0.0 0.0
>             0.5 0.5 0.0
>             2/3 1/3 0.0
>
> getden2   -1
> iscf2     -2
> tolwfr2   1.0d-12
> enunit2   1
>
> # Definition of the unit cell
> acell    2.443789213 2.443789213 15.0  Angstrom  ##########
> angdeg   90 90 60
>
> # Definition of the atom types
> ntypat 1          # Number of TYPEs of atoms
> znucl  6          # charge -Z- of the NUCLeus (atomic number)
>
> # Definition of the atoms
> natom     2              # total Number of ATOMs in the unit cell
> typat     2*1            # TYPE of atoms
> xangst
>     -0.00000     -0.00013      0.00000
>      1.22189      0.70580      0.00000
>
> ecut 12.0         # Maximal kinetic energy cut-off, in Hartree
> nstep   200       # Maximal number of SCF cycles
>



-- 
Pierre-Matthieu Anglade