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[Said Asma]

Dear PWScf Users,

I did the band structure of BN(zinc blende) with two atoms and I found aindirectgap  (like literature) Gamma-X. But when I did it with 8 atoms I found direct gap Gamma -Gamma.

Why such a problem occurs?

My input files is as following:
 
 *With TWO ATOMS:

cat > bn.band.in << EOF
 &control
    calculation='bands'
    pseudo_dir = '$PSEUDO_DIR/',
    outdir='$TMP_DIR/',
    prefix='bn'
 /
 &system
    ibrav=  2, celldm(1) = 6.76, nat=  2, ntyp= 2,
    ecutwfc = 60.0,
    occupations ='smearing',
    smearing = 'methfessel-paxton',
    degauss= 0.03,
 /
 &electrons
                 conv_thr = 1.0d-7 ,
                 mixing_mode = 'plain' ,
                 mixing_beta = 0.2 ,
                 diagonalization = 'david' ,
 /
ATOMIC_SPECIES
 B  10.811  B.pz-vbc.UPF
 N  14.007  N.pz-vbc.UPF
ATOMIC_POSITIONS
  B 0.00 0.00 0.00
  N 0.25 0.25 0.25
K_POINTS {crystal_b}
5
0.5  0.5  0.5  400
0    0    0    400
1.0  0    0    400
0.75 0.75 0    400
0    0    0    400
EOF
$ECHO "  running the band-structure calculation for bn...\c"
$PW_COMMAND < bn.band.in > bn.band.out
check_failure $?
$ECHO " done"



*and WITH 8 ATOMS:

cat > bn.band.in << EOF
 &control
    calculation='bands'
    pseudo_dir = '$PSEUDO_DIR/',
    outdir='$TMP_DIR/',
    prefix='bn'
 /
 &system
    ibrav= 1, celldm(1) = 6.82, nat=8, ntyp= 2,
    ecutwfc = 40.0, nbnd = 40,
    occupations ='smearing',
    smearing = 'methfessel-paxton',
    degauss= 0.02,
 /
 &electrons
  conv_thr =  1.0d-7,
    mixing_beta = 0.2,
 /

ATOMIC_SPECIES
 B  10.811  B.pz-vbc.UPF
 N  14.007  N.pz-vbc.UPF
ATOMIC_POSITIONS
 B      0.000     0.000     0.000
 N      0.250     0.250     0.250
 B      0.000     0.500     0.500
 N      0.250     0.750     0.750
 B      0.500     0.500     0.000
 N      0.750     0.250     0.750
 B      0.500     0.000     0.500
 N      0.750     0.750     0.250

K_POINTS {crystal_b}
4
0    0    0    40
0.5  0    0    40
0.5  0.5  0    40
0    0    0    40
EOF
$ECHO "  running the band-structure calculation for bn...\c"
$PW_COMMAND < bn.band.in > bn.band.out
check_failure $?
$ECHO " done"

Any help would be appreciated.
Best regards,
Said Asma
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