[Pw_forum] How to select unit cell for optimization calculation?

[max]

Hello,

I'm confusing, I need to do some convergence test, but I don't know if I
need primitive or one unit cell and how to select it.

I'm using this structure http://www.crystallography.net/cod/1513984.html

I extracted the crystallographic translation. The cell formula units are 8,
has 56 atoms.  See ATOMIC_POSITIONS bellow.

One unit cell of LiMn2O4 (spinel) has 7 atoms.

I have the following questions:



*Do I need to put 7 atoms in the primitive cell or how many?How can I
select the atoms position for primitive or one unit cell?*

ATOMIC_POSITIONS alat
   Li      0.125000000    0.125000000    0.125000000
   Li     -0.125000000    0.875000000    0.875000000
   Li      0.875000000   -0.125000000    0.875000000
   Li      0.875000000    0.875000000   -0.125000000
   Li     -0.125000000   -0.125000000   -0.125000000
   Li     -0.125000000    0.375000000    0.375000000
   Li      0.375000000    0.875000000    0.375000000
   Li      0.875000000    0.375000000    0.375000000
   Mn      0.500000000    0.500000000    0.500000000
   Mn     -0.500000000    1.250000000    1.250000000
   Mn      1.250000000   -0.500000000    1.250000000
   Mn      1.250000000    1.250000000   -0.500000000
   Mn     -0.500000000   -0.500000000   -0.500000000
   Mn      0.500000000   -0.250000000   -0.250000000
   Mn     -0.250000000    0.500000000   -0.250000000
   Mn     -0.250000000   -0.250000000    0.500000000
   Mn     -0.500000000    0.750000000    0.750000000
   Mn      0.000000000    1.250000000    0.750000000
   Mn      0.000000000    0.750000000    1.250000000
   Mn      1.250000000    0.000000000    0.750000000
   Mn      0.750000000   -0.500000000    0.750000000
   Mn      0.750000000    0.000000000    1.250000000
   Mn      1.250000000    0.750000000    0.000000000
   Mn      0.750000000    1.250000000    0.000000000
    O     -0.262700000    1.012700000    1.012700000
    O      1.012700000   -0.262700000    1.012700000
    O      1.012700000    1.012700000   -0.262700000
    O     -0.262700000   -0.262700000   -0.262700000
    O      0.262700000   -0.012700000   -0.012700000
    O     -0.012700000    0.262700000   -0.012700000
    O     -0.012700000   -0.012700000    0.262700000
    O      0.262700000    0.262700000    0.262700000
    O     -0.262700000    0.512700000    0.512700000
    O      0.237300000    1.012700000    0.512700000
    O      0.237300000    0.512700000    1.012700000
    O      1.012700000    0.237300000    0.512700000
    O      0.512700000   -0.262700000    0.512700000
    O      0.512700000    0.237300000    1.012700000
    O      1.012700000    0.512700000    0.237300000
    O      0.512700000    1.012700000    0.237300000
    O      0.512700000    0.512700000   -0.262700000
    O     -0.262700000    0.237300000    0.237300000
    O      0.237300000   -0.262700000    0.237300000
    O      0.237300000    0.237300000   -0.262700000
    O      0.262700000    0.487300000    0.487300000
    O      0.762700000   -0.012700000    0.487300000
    O      0.762700000    0.487300000   -0.012700000
    O     -0.012700000    0.762700000    0.487300000
    O      0.487300000    0.262700000    0.487300000
    O      0.487300000    0.762700000   -0.012700000
    O     -0.012700000    0.487300000    0.762700000
    O      0.487300000   -0.012700000    0.762700000
    O      0.487300000    0.487300000    0.262700000
    O      0.262700000    0.762700000    0.762700000
    O      0.762700000    0.262700000    0.762700000
    O      0.762700000    0.762700000    0.262700000

-- 
Máximo Ramírez
Physics Degree
Universidad Autónoma de Santo Domingo
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