  We present a lattice dynamics study of orthorhombic antimony sulphide (Sb2S3)
obtained using density-functional calculations in conjunction with the
supercell force-constant method. The effect of Born effective charges is taken
into account using a mixed-space approach, resulting in the splitting of
longitudinal and transverse optical (LO-TO) phonon branches near the zone
center. Zone-center frequencies agree well with Raman scattering experiments.
Due to the slow decay of the interatomic force constants (IFC), a minimal 2x4x2
supercell (Pnma setting) with 320 atoms is crucial for an accurate
determination of the dispersion relations. Smaller supercells result in
artificial acoustic phonon softening and unphysical lifting of degeneracies
along high symmetry directions. We propose a scheme to investigate the
convergence of the IFC with respect to the supercell sizes. The phonon
softening can be attributed to the periodic images that affect the accuracy of
the force constants, and the truncation of long-ranged forces. The
commensuration of the q-vectors with the supercell size is crucial to preserve
degeneracies in Sb2S3 crystals.
