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[ajlu@ustc.edu]

When I used unit cell with 8 atoms to calculate the electronic band structure 
of silicon, there are unexpected states within the band gap, at Gamma point. 
How to deal with it? 

My input file is attached below.

    ndtset   2
#Dataset 1 : usual self-consistent calculation
   kptopt1   1              # Kpoint option ( default 0.0 )
  nshiftk1   1              # Number of shifts of k point grids
   shiftk1   0.5  0.5  0.5  # These shifts will be the same for all grids
   prtden1   1              # Print the density, for use by dataset 2
   toldfe1   1.0d-6         # Tolerance on the difference of total energy
#Dataset 2 : the band structure
     iscf2  -2
   getden2  -1
   kptopt2  -3
    nband2   32
    ndivk2   10 12 17      # Number of divisions of k lines
kptbounds2   0.5  0.5  0.5 # L point
             0.0  0.0  0.0 # Gamma point
             0.0  0.0  0.5 # X point
             1.0  1.0  1.0 # Gamma point in another cell.
   tolwfr2   1.0d-12
   enunit2   1             # Will output the eigenenergies in eV
#Definition of the unit cell
    acell   3*10.217       # This is equivalent to   10.217 10.217 10.217
    rprim   1.0  0.0  0.0  # FCC primitive vectors (to be scaled by acell)
            0.0  1.0  0.0
            0.0  0.0  1.0
  chkprim   0
   ntypat   1              # There is only one type of atom
    znucl   14             # The atomic number of the type(s) of atom
    natom   8              # There are two atoms
    typat   8*1           # They both are of type 1, that is, Silicon.
     xred                  # The location of the atoms
            0.0  0.0  0.0  # Triplet giving the REDUCED coordinate of atom 1.
            0.5  0.5  0.0
            0.0  0.5  0.5
            0.5  0.0  0.5
            1/4  1/4  1/4
            3/4  3/4  1/4
            1/4  3/4  3/4
            3/4  1/4  3/4  # Triplet giving the REDUCED coordinate of atom 2.
     ecut   8.0            # Maximal kinetic energy cut-off, in Hartree
    nstep   10             # Maximal number of SCF cycles
   diemac   12.0           # Macroscopic dielectric constant diemac of the 
system


Thanks for your help.