[Pw_forum] LO-TO splitting

Stefano de Gironcoli degironc at sissa.it
Mon Feb 7 09:39:20 CET 2005

Fethi SOYALP wrote:

>Thank you very much for your help about LO-TO splitting in example06.
>Now I calculate vibration modes for a CaF2 structure.I am confused 
>because there isn?t degenerate some direcion, for egzample (110). How can
>I  determine which modes are LO-TO and which modes are LA-TA when
>degeneracy disapear. according to the output file, first two and last five
>eigenvectors are below. for first three eigenvectors, because of  the
>small modes, I can say first three modes are acoustic,  last six modes are
>optic .Near the X point (last four eigenvectors) acoustic and optic
>branches are crossing. I can?t determine acouistic  and optic modes from
>their greatnes. please gave me some comments. I have need your help.

away from high symmetry lines there may not be purely longitudinal and 
purely transverse modes, but all modes have mixed character.

In any case the DEFINITION of a longitudinal modes is that its 
eigenvector is parallel to the phonon vewavector, the DEFINITION of a 
transverse mode is that the eigenvector is orthogonal to it. Apply the 

best regards,

Stefano de Gironcoli

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