Band structure calculation: We used the Vienna Ab-Initio Simulation Package (VASP) to compute the electronic structure and optical spectra within first-principles density functional theory (DFT). The generalized-gradient approximation by Perdew, Burke, and Ernzerhof (PBE) was used to describe exchange and correlation and compared to the hybrid HSE06 functional by Heyd, Scuseria, and Ernzerhof. The projector-augmented wave (PAW) method was used to describe the electron-ion interaction and Kohn-Sham wave functions are expanded into a plane-wave basis up to a kinetic-energy cutoff of 500 eV.