An approach is presented for obtaining a highly accurate description of the energy-level alignment at hybrid interfaces, using quantum Monte Carlo calculations to include many-body effects as a correction to the standard single-particle picture. For a representative interface between an organic molecule and an inorganic slab, we illustrate the crucial role of many-body effects for correctly describing the energy-level alignment, leading to qualitatively different optoelectronic properties from

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... he prediction within the single-particle description. Further, the heterojunction behavior as a function of quantum confinement in the slab is predicted to be qualitatively different upon inclusion of many-body effects.