An analysis is presented that provides a density of states (DOS or D(E)) factor for Cs3Sb in the calculation of its quantum efficiency QE and emittance εn;rms using a Moments Approach. The analysis is based on density functional theory (DFT) adapted for the practical application of treating photoemission from bulk metal and semiconductor materials, and the interfaces between them. The Moments approach treats the processes of absorption, transmission and emission separately, for which DFT affects parameters and processes associated with each step, of which D, the optical constants n and k, and materials parameters such as effective mass mn and band gap Eg are paramount. Such factors are required to provide the components of an evaluation similar to the Tsu-Esaki formula for calculating current density over and through and over barriers, and will become more important when a proper quantum mechanical treatment of the emission barrier is considered beyond the simplistic thermal model (transmission probability is unity only for energy levels in excess of the barrier height and zero otherwise). Such features are expected to be far more consequential if the barrier supports resonant levels, e.g., heterostructures.