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Can surface phonons be calculated?

Surface phonon can be calculted from a supecell with a slab method. The slab should still preserve the three-dimensional periodic boundary condition. Perpendicular to the surface the supercell should have sufficient thick vacuum. The slab should be subjected to relevant space group, addapted to the studied surface. The lateral dimensions of the supercell should be large enough to account for reasonable interaction with the near surface neighbours. To avoid the winding of force constants between the neighbouring slabs, the vacuum thickness between slabs must be larger then the slab thickness itself. In practice the slabs cannot be choosen thick enough, so lattice relaxation is difficult to study. On the other hand one may find soft surface phonons, which indicate the surface reconstruction. Combining such a study with a change of surface lattice constants allows to study the surface changes due to stresses.