[Pw_forum] Ge band-gap

[Helen]

I have tried calculating the band-gap of Ge in a similar way to example01 for Si, and obtained a direct band-gap=0.75, indirect band-gap=0.46 at a lattice constant=10.614 (optimized lattice constant, converged  ecutwfc). At the experimental lattice constant the direct band-gap=0.51, indirect band-gap=0.38. In literature the band-gaps for LDA are quoted as being negative. Why am I getting positive results? Is there a problem with the pseudopotential? I noticed from the archives that in July 2004 this issue was raised, but an answer wasn't really given. Thank you very much for your help,

Dr. Helen Eisenberg,
Post-doctoral researcher,
The Fritz Haber Center for Molecular Dynamics,
The Hebrew University of Jerusalem,
Jerusalem 91904 Israel  

INPUT:

&control
    calculation = 'scf'
    restart_mode='from_scratch',
    prefix='ge',
    tstress = .true.
    tprnfor = .true.
    pseudo_dir = '$PSEUDO_DIR/',
    outdir='$TMP_DIR/'
 /
 &system    
    ibrav=  2, celldm(1) =10.6769, nat=  2, ntyp= 1,
    ecutwfc =27.0, 
 /
 &electrons
    diagonalization=cg
    mixing_mode = 'plain'
    mixing_beta = 0.7 
    conv_thr =  1.0d-8
 /
ATOMIC_SPECIES
 Ge  72.59  Ge.pz_bhs.UPF
ATOMIC_POSITIONS
 Ge 0.00 0.00 0.00 
 Ge 0.25 0.25 0.25 
K_POINTS
  10
   0.1250000  0.1250000  0.1250000   1.00
   0.1250000  0.1250000  0.3750000   3.00
   0.1250000  0.1250000  0.6250000   3.00
   0.1250000  0.1250000  0.8750000   3.00
   0.1250000  0.3750000  0.3750000   3.00
   0.1250000  0.3750000  0.6250000   6.00
   0.1250000  0.3750000  0.8750000   6.00
   0.1250000  0.6250000  0.6250000   3.00
   0.3750000  0.3750000  0.3750000   1.00
   0.3750000  0.3750000  0.6250000   3.00
EOF
    $ECHO "  running the scf calculation for Si...\c"
    $PW_COMMAND < ge.scf.cg.in > ge.scf.cg.out
    $ECHO " done"

    # band structure calculation along delta, sigma and lambda lines
    cat > ge.band.cg.in << EOF
 &control
    calculation='bands'                                            
    pseudo_dir = '$PSEUDO_DIR/',
    outdir='$TMP_DIR/',
    prefix='ge'
 /
 &system    
    ibrav=  2, celldm(1) =10.6769, nat=  2, ntyp= 1,
    ecutwfc =27.0, nbnd = 8,
 /
 &electrons                                                            
    diagonalization=cg
 /
ATOMIC_SPECIES
 Ge  72.59  Ge.pz_bhs.UPF
ATOMIC_POSITIONS
 Ge 0.00 0.00 0.00 
 Ge 0.25 0.25 0.25 
K_POINTS
 6
   0.0 0.0 0.0 1.0
   0.1 0.1 0.1 1.0
   0.2 0.2 0.2 1.0
   0.3 0.3 0.3 1.0
   0.4 0.4 0.4 1.0
   0.5 0.5 0.5 1.0

EOF:


RESULTS:

         k = 0.0000 0.0000 0.0000     band energies (ev):

    -7.1507   5.4288   5.4288   5.4288   5.9790   8.0294   8.0294   8.0294

          k = 0.1000 0.1000 0.1000     band energies (ev):

    -7.0370   3.6667   5.2082   5.2082   6.8127   8.3334   8.3334   8.6989

          k = 0.2000 0.2000 0.2000     band energies (ev):

    -6.7011   1.6617   4.7838   4.7838   6.5618   8.8874   8.8874  10.4632

          k = 0.3000 0.3000 0.3000     band energies (ev):

    -6.1690  -0.1311   4.4065   4.4065   6.1730   9.1840   9.1840  12.2437

          k = 0.4000 0.4000 0.4000     band energies (ev):

    -5.5394  -1.5375   4.1640   4.1640   5.9087   9.1822   9.1822  12.7544

          k = 0.5000 0.5000 0.5000     band energies (ev):

    -5.1829  -2.1557   4.0814   4.0814   5.8180   9.1444   9.1444  12.6631
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