[Pw_forum] Error band structure

[Said Asma]

 Dear Giannozzi ,

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 (the unit cell has changed to simple cubic).
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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