[Pw_forum] LO-TO splitting

[Stefano de Gironcoli]

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 
definitions.

best regards,

Stefano de Gironcoli