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[Aaron Zhou <fishzhou@gmail.com>]

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

Attachment: graphene vacuum.ppt
Description: MS-Powerpoint presentation