[Pw_forum] bandgap of germanium

[华 李]

Dear Paolo:
       I have calculated the band structure with lattice constant 5.64613 (A) , and set  calculation = 'relax'. Then I get the band gap of the germanium 0.2269 eV. It's  also too large.
      I tried to optimize band structure, just like CO in example3.
input file :
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
       &control
    calculation = 'relax'
    restart_mode='from_scratch',
    prefix='germaniun',
    tstress = .true.
    tprnfor = .true.
    pseudo_dir = '/usr/people/lty/code/pseudo//',
    outdir='/usr/people/lty/tmp//'
 /
 &system
    ibrav=  0, nat=  2, ntyp= 1,
    ecutwfc =30.0, nbnd = 8
 /
 &electrons
    diagonalization='cg'
    mixing_mode = 'plain'
    mixing_beta = 0.7
    conv_thr =  1.0d-8
 /
 &ions
    upscale = 10.0
 /
ATOMIC_SPECIES
 Ge  72.59  Ge.pz-bhs.UPF
ATOMIC_POSITIONS
 Ge 0.00 0.00 0.00 1 1 1
 Ge 0.25 0.25 0.25 1 1 1
CELL_PARAMETERS
 10.0 00.0 00.0
 00.0 10.0 00.0
 00.0 00.0 10.0
K_POINTS
  6
   0.0  0.0  0.0   1.00
   0.1  0.1  0.1   1.00
   0.2  0.2  0.2   1.00
   0.3  0.3  0.3   1.00
   0.4  0.4  0.4   1.00
   0.5  0.5  0.5   1.00
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
 Then I get the band gap :
***********************************************************************

          k = 0.0000 0.0000 0.0000 ( 2777 PWs)   bands (ev):
   -11.1883  -7.1028  -2.6627  -1.6724  -1.6724  -0.1296  -0.1296   4.2633
          k = 0.1000 0.1000 0.1000 ( 2785 PWs)   bands (ev):
   -11.1650  -7.1625  -2.5315  -1.8738  -1.8738   0.1408   0.1408   2.0930
          k = 0.2000 0.2000 0.2000 ( 2785 PWs)   bands (ev):
   -11.1051  -7.2871  -2.3407  -2.2155  -2.2155   0.5997   0.5997   1.8475
          k = 0.3000 0.3000 0.3000 ( 2785 PWs)   bands (ev):
   -11.0338  -7.3931  -2.4812  -2.4812  -2.3644   0.7896   0.7896   2.4967
          k = 0.4000 0.4000 0.4000 ( 2764 PWs)   bands (ev):
   -10.9785  -7.4498  -2.6302  -2.6302  -2.4865   0.6643   0.6643   3.4208
          k = 0.5000 0.5000 0.5000 ( 2800 PWs)   bands (ev):
   -10.9579  -7.4662  -2.6758  -2.6758  -2.5383   0.5696   0.5696   3.0471
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
The band-gap is -0.8659 eV. I think the band-gap is error. Which must be adjusted in my input-file, then I get the right band-gap ?
 
Best wishes
 
tylv

Paolo Giannozzi <giannozz at nest.sns.it> wrote:
On Saturday 31 July 2004 11:04, Bachir Bouhafs wrote:

> [...] I think that you have calculated your band structure at the
> experimental lattice parameter [...] which is greater than the LDA 
> value. Which means, that you have applied a negative pressure 
> to your crystal, and consequently the band gap increases.

[ please do not post HTML ]

and this is likely to be the main reason of the discrepancy. Other 
factors, such as the pseudopotential, cutoff, number of k-points, 
may contribute to the difference. For instance: in GaAs, the use 
of a nonrelativistic pseudopotential for Ga increases the band gap
by approx. 0.4 eV.

By the way: the tests and examples are NOT meant to give any
realistic description of any realistic systems. 

Paolo
-- 
Paolo Giannozzi e-mail: giannozz at nest.sns.it
Scuola Normale Superiore Phone: +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7 I-56126 Pisa, Italy
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