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Mar9-12, 11:51 AM
Hmm I'm not quit sure I understand. I have the vibrational modes
Vibration = 2 * E1u + 2 * E2g + 2 * A2u + 2 * B1g
Summing the dimensions I get 12 modes, I guess that must be 9 optical and 3 acoustic modes (4 atoms per unit cell should give 12 modes), so to me it seems like the acoustic modes are included.
I have attached a dispersion relation for graphite, it seems like on that picture that one have
Optical = E1u + 2*E2g + 2*B1g + A2u
Acoustic = E1u + A2u
Well, that is correct, translations of the crystal in plane (x y) transform like E1u and along z like A2u. However, they aren't vibrations in the strict sense.