I have a couple of conceptual questions regarding the calculation of phonon dispersions. I wish to calculate the phonon dispersion of Nickel with my lattice vectors given by [111], [1-10] and [11-2] (though my lattice vectors are non-orthogonal to minimize the number of atoms. My lattice vectors are 2.48901587 0 0, 1.244507935 2.155550974 0, 0 0 40). I also have ~15 A vacuum in my cell. I seem to be getting incorrect phonons for this structure but my results are correct if I choose a FCC unit cell with 4 atoms. My questions are summarized below: Do I have to make any changes to my calculations because I have vacuum in my cell?I see that calculating force constants (using small displacement method), and hence phonons, requires the creation of supercells. What should they ideally be? Would it be wrong if I just use 1x1x1 cell?I have attached the results I'm obtaining. The first figure is for the Ni unit cell and the second figure is for the Ni cell with vacuum.
Calculation of phonon dispersion
density functional theory