[Pw_forum] Error band structure

[Said Asma]

 Dear PWScf Users,

I did the band structure of  BN (zinc blende) with two atoms and I found a indirect gap  (like literature) Gamma-X. But when I did it with 8 atoms I found direct gap Gamma -Gamma.
Normally if we change (nat = 2, ibrav = 2) to (nat = 8, ibrav = 1),we find the same nature of gap.
If such we have the composition B0.25Ga0.75N how we can determine the nature of gap?

 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



bests

Said Asma



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