[Pw_forum] Electronic band structures for primitive and conventional unit cells

[pascal boulet]

Thank you all for your responds.

  We indeed knew about the folding of the BZ when using conventional 
cells, but we overlook the possibility that X point becomes Gamma point.

Best regards,
Pascal



Le 18/03/2013 23:30, Yun Wang a écrit :
> Dear Pascal,
>
>   What you found is related to the Brillouin zone folding. You can 
> find more details in Roarld Hoffmann's book: Solids and surfaces. The 
> band structure using the primitive cell is correct.
>
> Cheers,
> Yun
>
> On Mon, Mar 18, 2013 at 9:05 PM, pascal boulet 
> <pascal.boulet at univ-amu.fr <mailto:pascal.boulet at univ-amu.fr>> wrote:
>
>     Dear all,
>
>     We are facing a problem with differences in electronic band
>     structures when we use primitive or conventional unit cells.
>
>     The structure we are dealing with is Mg2Si for which the primitive
>     UC is:
>     ATOMIC_POSITIONS crystal
>     Si       0.000000000   0.000000000   0.000000000
>     Mg       0.250000000   0.250000000   0.250000000
>     Mg       0.750000000   0.750000000   0.750000000
>
>     The cubic space group is 225, a=12.0421785 bohr and Mg2Si is FCC.
>
>     The rest of the input is (I make it short):
>     K_POINTS crystal
>     120
>             0.5000000000    0.2500000000    0.7500000000    1.0
>             0.5000000000    0.2600000000    0.7400000000    1.0
>             0.5000000000    0.2700000000    0.7300000000    1.0
>      etc.
>      calculation   = 'bands',
>       celldm(1)   =12.0421785,
>       nat         = 3,
>       ibrav       = 2,
>
>
>     If we use this UC we get a indirect band gap (G-X), which is
>     correct. If we now use the conventional UC (12 atoms in the cube)
>     we get a direct band gap at Gamma. In this case, we are using the
>     following input file :
>       calculation   = 'bands',
>       celldm(1)   = 1,
>       nat         = 12,
>       ibrav       = 0,
>
>     K_POINTS crystal
>     126
>     .0   .0   .0   1.
>     .020 .020 .020 1.
>     .040 .040 .040 1.
>     .060 .060 .060 1.
>     .080 .080 .080 1.
>     .100 .100 .100 1.
>     .120 .120 .120 1.
>     etc.
>
>     CELL_PARAMETERS
>     12.03517200     0.0      0.0
>     0.0      12.03517200     0.0
>     0.0      0.0      12.03517200
>     ATOMIC_POSITIONS crystal
>     Si    0.00000    0.00000    0.00000
>     Si    0.50000    0.50000    0.00000
>     Si    0.00000    0.50000    0.50000
>     Si    0.50000    0.00000    0.50000
>     Mg    0.25       0.25       0.25
>     Mg    0.75       0.75       0.75
>     Mg    0.25       0.25       0.75
>     Mg    0.25       0.75       0.25
>     Mg    0.75       0.25       0.25
>     Mg    0.25       0.75       0.75
>     Mg    0.75       0.25       0.75
>     Mg    0.75       0.75       0.25
>
>     Could you please tell us what we are doing wrong in the second
>     calculation?
>
>     Thank you for your response.
>
>     Best regards,
>     Pascal
>
>
>
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>
>
>
> -- 
>
> Dr. Yun Wang
> Research Fellow
> Centre for Clean Environment and Energy
> Griffith School of Environment
> Gold Coast Campus, Griffith University
> QLD 4222, Australia
> Tel:(61-7) 5552 8456
> Fax:(61-7) 5552 8067
>
>
>
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