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Solar cells based on hybrid organic-inorganic halide perovskites have become the most promising contenders in the race for maximum power conversion efficiency (PCE). Easy to process and possessing wide chemical tunability, the reasons for their remarkable PCEs of >22% are not fully understood yet. However, the electronic structure and excited state properties of materials like methylammonium lead iodide, CH₃NH₃PbI₃, certainly play a key role in their success. First principles calculations have contributed both to the understanding of the optoelectronic properties of halide perovskites, and to the prediction of new, more stable, and environmentally benign materials. While density functional theory (DFT) is a standard tool to calculate ground state properties accurately, it is insufficient for the calculation of excited states. DFT eigenvalues and orbitals can, however, serve as starting points for Green’s function based many-body perturbation theory (MBPT) which allows to obtain quasiparticle spectra, band gaps, and optical absorption spectra. In my talk, I will address the predictive power of these first principles methods for halide perovskites. By studying a set of halide perovskites representative of the electronic diversity of this broad class of materials, I will show that even computationally expensive Green’s function-based methods lack predictive power for these systems. The consequences of these findings for the quantitative prediction of excited state and other electronic structure properties will be discussed.