[Pw_forum] Error band structure

[Peng Tao]

Dear Said,
It is not an error. When you enlarge the primitive cell, the  brillouin zone gets smaller. Moreover, some high symmetric points might be overlapped with Gamma, for the symmetry is changed (ibrav changes). This effect is called  brillouin zone folding.

Plato Tao

-----Original Messages-----
From: "Said Asma" <saidasma1987 at yahoo.fr>
Sent Time: Thursday, April 26, 2012
To: "giannozz at democritos.it" <giannozz at democritos.it>, "pw_forum at pwscf.org" <pw_forum at pwscf.org>
Cc:
Subject: [Pw_forum] Error band structure






 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 changedtosimplecubic).
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



________________________________







--
-------------------------------------------------------------------
PH.D. candidate Peng Tao 
Magnetism and Magnetic Materials Division
National Laboratory for Material Science
Institute of Metal Research, Chinese Academy of Sciences
Phone  +86-024-83978751
-------------------------------------------------------------------



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