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

[pascal boulet]

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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