Try to read this: http://www2.chem.umd.edu/groups/vedernikov/Lecture2009-10-02.pdf

Dear Alexander Krylov,

Thank you very much for giving answer.

I want to know the optical and acoustical modes and I have to calculate the these modes by phonon spectrum using PwScf.

Could you send the any pdf or video lecture on this topic.

Thank you.

@Gangaprasad, Hi, How are you? A or B represents that wave functions are singly degenerate. If E or T is present it suggests doubly or triply degenerate wave functions. Finally the symbols u and g denotes the type of the wave function if it is u-Ungerade(Odd), g-gerade (even). I hope you received the answer for your query.