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Dear Said<br>
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.<br>
In particular Gamma plus the 3 X points of the FCC BZ are folded
into Gamma of the SC.<br>
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.<br>
Best regards,<br>
<br>
stefano<br>
<br>
<br>
On 04/25/2012 06:07 PM, Said Asma wrote:
<blockquote
cite="mid:1335370074.23583.YahooMailNeo@web29007.mail.ird.yahoo.com"
type="cite">
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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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