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Accurately modeling molecular crystal polymorphism requires careful treatment of diverse intra- and intermolecular interactions which can be difficult to achieve without the use of high-level ab initio electronic structure techniques. Fragment-based methods like the hybrid many-body interaction QM/MM technique enable the application of accurate electronic structure models to chemically interesting molecular crystals. The theoretical underpinnings of this approach and the practical requirements for the QM and MM contributions are discussed. Benchmark results and representative applications to aspirin and oxalyl dihydrazide crystals are presented.