Each band within this manifold is characterized by a certain, albeit smeared, value of k z in the Fourier expansion of the corresponding wave-function. The relation between such band structure calculations and ARPES spectra, which is far from obvious, can be recovered using so-called "band unfolding". In this approach, the band structure generated from supercell calculations is projected, or unfolded, onto the appropriate smaller unit cell. The band manifolds in the small Brillouin zone of the supercell then split into the separate bands in the larger Brillouin zone of the smaller unit cell, untangling the spaghetti. This can help resolve e.g., the contributions of surface states vs. the bulk bands of the material. In the bulk Brillouin zone, the surface-perpendicular Γ-K|U ′ -X ′ direction is symmetry equivalent to the surface-parallel Γ -K|U ′ -X ′ directions, as shown in Figure via the blue and red arrows respectively. However, the finite length of the slab in the surfaceperpendicular direction breaks this symmetry and produces a Brillouin zone that is very thin in the surface-perpendicular direction. This manifests in the differences between the unfolded band structures along the surface-parallel vs. the surface-perpendicular paths, shown in origin of the discrete broadened energy levels, is provided in the SI).