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Dear Developers,

I am trying to calculated phonon frequencies at the Gamma point. I understand this can be done using tag IBRION=5 or 6 with the difference that the first one does not include any symmetry considerations.

What I do not understand is that my result with IBRION=6 depends also on the used ISIF tag (even if NSW=0). In particular, for IBRION=6 and ISIF=2 I get similar results (up to numerical noise, I suppose) with IBRION=5, but when using ISIF=4 or ISIF=6 (to get simultaneously also the elastic constants), the obtained frequencies differ. Why?

Thanks,
David

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