[Pw_forum] IFCs and Difference between unit cell and supercell

Paolo Giannozzi giannozz at democritos.it
Sat Oct 20 14:07:28 CEST 2012

On Oct 20, 2012, at 12:59 , karan deep wrote:

> still vibrational modes are different

they aren't

> for sc it gives
>      omega(  1 -  3) =          9.5  [cm-1]   --> T_2  G_15 P_4   I+R

this is the acoustic (zero-frequency) mode

>      omega( 22 - 24) =        510.2  [cm-1]   --> T_2  G_15 P_4   I+R

this is the optical mode. Al other zone-center modes of the sc lattice
do not corrspond to zone-center modes of the fcc lattice.

> for fcc it gives

the same (zero-frequency mode do not have exactly zero frequency
for reasons that have been explained too many times)

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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