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I am currently calculating phonon dispersion curve of silicon carbide using Phonon and VASP5.2. But the two optical phonons degenerate at gamma point. However, in the literatures I currently have, the optical phonons should be separated. So I tried to introduce LO-TO splitting in the calculation. But the shape of phonon dispersion curve looked strange when I did that.
Is this an internal problem with direct method (small displacement method)? How can I get the gamma point separated in phonon dispersion curve? Or is there any other advice?
Thank you very much!


<span class='smallblacktext'>[ Edited ]</span>

As far as I can see, you should have 3 optical phonon branches at the Gamma-point, of which 2 are degenerate (at the Gamma-point). It might help to see how the branches look when you approach the Gamma-point from different directions.

[quote="maartendft"]As far as I can see, you should have 3 optical phonon branches at the Gamma-point, of which 2 are degenerate (at the Gamma-point). It might help to see how the branches look when you approach the Gamma-point from different directions.[/quote]

Yes, I do have three optical branches. But instead of two, all three of them degenerate at gamma point. Is there anything I can do to remove the degeneracy? I tried to add in LO/TO splitting, but it didn't work well.

How did you introduce the LO-TO splitting in the calculation?
I have got the phonon frequency and Born effective charge by Vasp,but I don't know how to introduce the Born effective charge to the phonon frequency.