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Phonons in crystals with strong phonon-magnetic coupling.

If you have a crystal with some antiferromagnetic structure, and coupling of magnetic moments to atomic distortions is not negligeble, then phonon dispertion curves can be entirely wrong, if antiferromagnetic structure is not considered. Suppose that the crystallographic structure belongs to high-symmetry space group Fd-3m, and remain such if the magnetic structure is ferromagnetic. In antiferromagnetic configuration up and down atoms, may not equivalently interact with the neighbors, It could be so bad that there appear faith soft modes. Therefore, to establishe proper space group one should name the atoms with spin up and down as two spicies, hence in Phonon the Names should be different. The relevant low-symmetry space group can be found using the FindSym program supplemented to Phonon. One may find then that one of the Fd-3m can be I4_1/amd space subgroup. The Phonon calculations should be performed in I4_1/amd. As a consequence the list of non-equivalent atoms displacements used to derive the Hellmann-Feyman forces increases. If you are lucky, then this approach may remove the undesired soft modes.