Notwithstanding the great development of theoretical methods, an accurate treatment of extended chemical systems remains one of the great challenges. While practical chemistry -be it synthesis, analysis, structure investigation -is usually carried out in the condensed phase, computational chemistry prefers the more convenient gas phase description. Although the environmental influence may be small or favourably cancel out, it is more often the case that the properties of molecules are impacted by the phase change. The environment may significantly modify molecular properties like molecular vibrations, excitation/emission energies, or NMR chemical shifts. Matters become even more complicated for the very same molecule being able to form different packings, known as polymorphs. These may be related to different conformations of the molecule and can be in very close energy range, but still have severe impact on solubility or even more day-to-day concerns like the taste of chocolate. Great progress has been made in both the accurate description of single crystals as well as the efficient screening and ranking of multiple candidate structures for the prediction of unknown molecular crystal structures. Proof of that is visible in the results of the Cambridge Crystallographic Database blind test challenge. Nowadays there exists a sleuth of methods for these types of calculations. These fall under two main categories. On the one hand, crystals can be studied by applying inherently periodic methods which describe true Bloch states. The most popular methods are based on planewave DFT, which is readily available in a number of codes, but DFT applications with Gaussian basis sets exist as well. However, extensions of such methods beyond pure DFT can be troublesome. The computational demand of exact Hartree-Fock (HF) exchange for hybrid DFT in periodic calculations already limits the applicability compared to general gradient approximation (GGA) and meta-GGA functionals. Post HF wave function theory, as routinely used in non-periodic calculations, is even more cumbersome to apply or even implement, albeit the progress made in recent years through the use of local correlation. On the other hand, fragmentation approaches have been put forward which aim to describe the infinite crystal through a careful selection of energy terms. Such expansions can be used for the total energy or solely for electronic correlation. A limited number of molecules is calculated at the QM level together to capture intermolecular interactions among them, and the results of multiple calculations are added up to approach the crystal limit. Since the QM region is now limited, some of these methods can, in principle, directly make use of any gas phase QM method in existence -the very same basis sets, approximations, and corrections. However, these approaches require a reasonable modeling of the environment or require expanding to quite a large size in order to describe many-body interactions. One can also view subtractive quantum mechanics / molecular mechanics (QM/MM) formulations as one variation of these models, correcting a low-level periodic calculation by the difference to a high-level approach computed for isolated molecules. Missing in the list above is one of the prime methods used in the study of non-ordered condensed phase molecular systems: additive QM/MM. Albeit dominant in the study of enzymatic reactivity and other chemical phenomena in solution, it has yet to find an established place in the field of molecular crystals. The reasons are manifold, but particularly severe is the question of how to handle the inherent symmetry of a crystal within such a setup, a problem which is circumvented in subtractive schemes by neglecting embedding effects in the QM corrections. The methods known in the literature so far which could come close to an additive QM/MM formulation are relatively few. The EMBE model of Manby and coworkers makes use of approximate potentials, avoiding the use of molecular mechanics altogether. So far, no analytic gradients are available. The crystal-QM/MM model of Mörschel and Schmidt 36 uses point charges and parametrised 6-exp potentials, but does only allow for an optimisation of the cell parameters. Finally, Bjornsson and Bühl 37 presented a basis for additive QM/MM with a fixed environment, which we discuss in greater detail in the Method Section.