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[<shinekin@hotmail.com>]

Hi Dear all,

In the band structure calculation of SnO2, I want to evaluate the dependence 
of
band gap on the exchange-correlation potential.

I found that the GGA underestimated the band gap substantially; the calculated
value was only 0.82 eV, but the LDA give a value of 2.9eV while

the experimental value was 3.6eV. Can anyone tell me why this two potentials
given a totally different results? Why the LDA gets so

close to the experimental result while The GGA not? 

Any suggestion or advice would be greatly appreciated! Thanks in advance.

Below is the detail of calculation: (Except the exchange-correlation 
potential,
every parameter keeps unchanged)

----------------
      
ndtset 2

#definition of unit cell
acell
      8.6056921037E+00  8.6056921037E+00  5.6698618700E+00
rprim
     1 0 0
     0 1 0
     0 0 1
# definition of atom
ntypat 2
typat 1 1 1 1 2 2
natom 6
znucl 8 50
# xred
#    0.000000   0.000000   0.000000   # sn1
#    0.500000   0.500000   0.500000   # sn2
#    0.307000   0.307000   0.000000   # o1
#   -0.307000  -0.307000   0.000000   # o2
#    0.193000   0.807000   0.500000   # o3
#    0.807000   0.193000   0.500000   # o4

xred          3.0407683302E-01  3.0407683302E-01  0.0000000000E+00
             -3.0407683302E-01 -3.0407683302E-01  0.0000000000E+00
              1.9592316698E-01  8.0407683302E-01  5.0000000000E-01
              8.0407683302E-01  1.9592316698E-01  5.0000000000E-01
              0.0000000000E+00  0.0000000000E+00  0.0000000000E+00
              5.0000000000E-01  5.0000000000E-01  5.0000000000E-01

# definition of scf 
kptopt1 1
ngkpt1 6 6 6
prtden1 1
toldfe1 1.0d-6


# band calculation
iscf2 -2
getden2 -1
tolwfr2 1.0d-12
kptopt2 -7
nband2 28
prteig2 1
ndivk2 6 6 6 6 4 6 4

kptbounds2
           0.0 0.0 0.5   # Z
           0.5 0.5 0.5   # A
           0.5 0.5 0.0   # M
           0.0 0.0 0.0   # G
           0.0 0.0 0.5   # Z
           0.0 0.5 0.5   # R
           0.0 0.5 0.0   # X
           0.0 0.0 0.0   # G

ecut 42
enunit2 0       # in hatree
#-----------------------------------

nstep 100

dilmac 12