# [Pw_forum] IFCs and Difference between unit cell and supercell

karan deep karandeepster at gmail.com
Sat Oct 20 12:59:07 CEST 2012

```> When I use supercell: simple cubic with 8 atoms to build silicon
> structure, the omega is different at gamma point with the omega from
> the structure with FCC.

This is same kind of question i asked about AFM material. sc is giving
8x3=24 modes of vibrations.
fcc gives 2x3=6 modes and these are different.

>in order to get exactly the same number, you have to use either
>a converged grid of k-points, or exactly the same k-points.

i try the example 2 of silicon which has 10 k-points mentioned in it for
both sc and fcc.

still vibrational modes are different. for sc it gives

Mode symmetry, T_d (-43m)  point group:

omega(  1 -  3) =          9.5  [cm-1]   --> T_2  G_15 P_4   I+R
omega(  4 -  6) =        140.4  [cm-1]   --> T_1  G_25 P_5
omega(  7 -  9) =        140.7  [cm-1]   --> T_2  G_15 P_4   I+R
omega( 10 - 12) =        408.2  [cm-1]   --> A_1  G_1  P_1   R
omega( 10 - 12) =        408.2  [cm-1]   --> E    G_12 P_3   R
omega( 13 - 15) =        408.3  [cm-1]   --> T_2  G_15 P_4   I+R
omega( 16 - 18) =        458.4  [cm-1]   --> T_1  G_25 P_5
omega( 19 - 21) =        458.5  [cm-1]   --> T_2  G_15 P_4   I+R
omega( 22 - 24) =        510.2  [cm-1]   --> T_2  G_15 P_4   I+R

for fcc it gives

Mode symmetry, O_h (m-3m)  point group:

omega(  1 -  3) =          3.2  [cm-1]   --> T_1u G_15  G_4- I
omega(  4 -  6) =        510.2  [cm-1]   --> T_2g G_25' G_5+ R

any suggestions please

Karandeep
Research Scholar
Physics Department,
IIT,  Delhi
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