In addition to its potential relevance for different applications, highlighted above, 5-FU adsorption in zeolites is also interesting from a more fundamental point of view. Due to the presence of two negatively polarised carbonyl oxygen atoms and two amine groups (Figure ), 5-FU can act as hydrogen bond donor and acceptor, and a stabilisation in the zeolite pores through various hydrogen bonds can be envisaged, especially for protonic zeolites having (at least) two framework protons in relatively close proximity. Experimentally, the nature and extent of such "multi-site" interactions is rather difficult to probe, not least due to the important influence of defects on the adsorption properties, as highlighted above when discussing the work of Datt et al. In contrast, atomistic modelling methods are ideally suited to investigate such interactions in a systematic fashion, giving access to adsorption energies, low-energy structures, and derived quantities like vibrational spectra. Molecular dynamics simulations allow to study the dynamic behaviour of the host-guest system. It is clear that the use of idealised models of perfect zeolite crystals constitutes a certain limitation, as not all features that may play a role in real materials (defects, intrinsic mesoporosity, …) are incorporated in the modelling. So far, atomistic modelling methods have been employed to study the adsorption of pharmaceuticals in zeolites, both in the context of "screening" studies addressing a range of compounds in different zeolites with computationally efficient force field (FF) methods and in studies of individual compounds using computationally more expensive density functional theory (DFT) calculations. The specific topic of 5-FU adsorption in zeolites has so far received limited attention from the computational perspective, the previously mentioned study of 5-FU diffusion by Spanakis et al. being an exception. However, it is worth mentioning that dispersion-corrected DFT has been used to investigate the adsorption of 5-FU at a hydroxylated cristobalite surface. That work highlighted the important role of hydrogen bonds in stabilising the surface-adsorbed 5-FU molecule. Moreover, a DFT-based composite electronic structure method was employed to shed light on the local structure and vibrational properties of 5-FU adsorbed in HKUST-1, a copper-containing MOF. In the present work, dispersion-corrected DFT calculations are employed to investigate the adsorption of 5-FU in FAU-type zeolites, considering aluminosilicate models having different amounts of Al atoms and charge-balancing framework protons as well as a reference all-silica FAU model. Focus is placed on FAU for various reasons: First, FAU-type zeolites have been addressed in several previous drug delivery investigations, as highlighted above. Second, they are available across the whole range of Si/Al ratios (FAU type systems with Si/Al < 1.5 are commonly designated "zeolite X", those with Si/Al 1.5 are dubbed "zeolite Y"), whereas many other framework types are synthetically accessible only within a certain range. Finally, the relatively simple structure, with only one non-equivalent T site, renders the generation of models having different proton arrangements relatively straightforward. The scope of the present study remains restricted to protonic, rather than cationic zeolites because the propensity of metal cations to associate with 6MRs of the FAU structure renders interactions of one 5-FU molecule with several cations unlikely, except, potentially, at high cation concentrations (approaching a Si/Al ratio of 1). The aims can be summarised as follows:  The possible extent of "multi-site" interactions is probed through calculations for FAU models with different proton arrangements. In addition to comparing adsorption energies, an analysis of the low-energy configurations gives insights into the role of hydrogen bonds.