[Pw_forum] Error band structure

[Stefano de Gironcoli]

Dear Said
   when you use a supercell many k-point of the original BZ are folded 
in the same point of the supercell BZ.  If you use a SC supercell for a 
crystal that would have an FCC Bravais lattice (using a cell which has a 
4 times larger volume) 4 kpoints of the FCC BZ are folded in any point 
of the SC BZ.
   In particular Gamma plus the 3 X points of the FCC BZ are folded into 
Gamma of the SC.
   A good exercise would be to compare the band structure of the two 
equivalent systems and find the correspondence of each band of the FCC 
in the band structure of the SC. You should find perfect matching.
   Best regards,

stefano


On 04/25/2012 06:07 PM, Said Asma wrote:
>
>
>
>   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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