[Pw_forum] bandgap calculation of insulators with direct gap

[Amin Torabi]

Dear Paolo,

Using the following input, I did two calculations, once at gamma point
only, and once with 4 4 3 0 0 0 K-sampling, to calculate the bandgap of ZnS
(a semiconductor with a direct gap at gamma point)

/
&SYSTEM
        ibrav      = 4
        A           = 3.811
        C           = 6.234
        nat         = 4
        ntyp       = 2
        nbnd      = 19
        ecutwfc  = 80
/
&ELECTRONS
/
ATOMIC_SPECIES
 Zn 65.409  Zn.pbe-nc.UPF
 S  32.066  S.pbe-n-nc.UPF
ATOMIC_POSITIONS crystal
 Zn  0.33333 0.66667 0.000
 Zn  0.66667 0.33333 0.500
 S   0.33333 0.66667 0.375
 S   0.66667 0.33333 0.875
K_POINTS automatic        OR     K_POINTS gamma
 4 4 3 0 0 0


For gamma only calculation, I have the following band energies:

 k = 0.0000 0.0000 0.0000 (  3194 PWs)   bands (ev):
    -5.6961  -4.6015  -0.1027  -0.1027  -0.0711  -0.0495  -0.0495   0.2414
     0.2414   0.3243   0.3243   0.5610   1.6809   6.5472   6.5474   7.4256
     7.5926   7.5926   9.2391
     highest occupied, lowest unoccupied level (ev):     7.5926    9.2391


For 4 4 3 0 0 0 sampling, I got the followings:

k = 0.0000 0.0000 0.0000 (  6387 PWs)   bands (ev):
    -6.6872  -5.7232   0.0468   0.0574   0.0574   0.1373   0.1373   0.4912
     0.4913   0.5680   0.5680   0.7323   1.1728   5.6554   5.6556   6.5171
     6.5888   6.5888   8.6817
     highest occupied, lowest unoccupied level (ev):     6.5888    8.6817



In both cases the gap occurs at gamma point, however, the numbers are
different. How should I explain this?

Thanks in advance,



On Wed, Dec 19, 2012 at 5:41 AM, Paolo Giannozzi <giannozz at democritos.it>wrote:

>
> On Dec 19, 2012, at 11:28 , Giuseppe Mattioli wrote:
>
> > Always negligible experimental measurements (i.e., vox populi)? :-)
>
> experiments are made on real semiconductors, not on theoretical
> ones ...
>
> P.
> ---
> Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
>
>
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>



-- 
***************************************
Amin Torabi
Ph.D. student
Chemistry Dept.
The University of Western Ontario
London, On Canada, N6A 5B7
Phone: 519-661-2111 Ext: 87871
****************************************
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