The computational part of this study reveals that the selectivity in the ALD process stems from the chemoselective adsorption of Hacac on the NGS (Al2O3) w.r.t. the GS (SiO2). Interestingly, this is a common theme in AS-ALD between two dielectric surfaces, the inhibitor chemisorbs on both surfaces. As shown in Fig. ., the condensation reaction with the surface-OH groups leads to elimination of H2O and the formation of a bidentate (chelate) binding mode on both surfaces. However, selectivity stems from two factors now. First, a thermodynamic driving force is found on the NGS (ΔE = -47 kJ•mol -1 , Fig. .a) while the reaction is unfavorable on the GS (ΔE = 95 kJ•mol -1 , Fig. .b). Second, and more importantly, the reaction barriers vastly differ. While a barrier of only ΔE ‡ = 24 kJ•mol -1 is found on the NGS, it is 227 kJ•mol -1 on the GS. Notably, both reactions start with a weakly bound Hacac species on the surface (Fig. in the SI of ref. ). These states are called 'precursor-states' in the surface science literature (this term will not be used further to avoid confusion with the ALD usage of the term 'precursor'), and are often physisorbed, which means they are bound only by London dispersion forces without a significant covalent bonding contribution (other definitions of physisorption exist but this one is supported by quantitative bonding analysis ). The next step in the reaction mechanism is the formation of an intermediate species (yellow dots in Fig. .) which shows hydrogen-bonding of the inhibitor to the NGS and physisorption for the GS. Why do these species differ so much although both surfaces are OH-terminated? The reason is the different orientation of the OH-groups which is pointing towards the inhibitor (NGS) or away from the inhibitor (GS). This is one example, where the detailed arrangement of surface atoms is decisive for the selectivity of inhibitor adsorption which has been discussed in detail in the supporting information of this study. However, it also shows the care that needs to be taken to select appropriate surface models (section 2.8) since other facets in the modelling could show different arrangements of these OH-groups. One feature here, which is also observed in other studies, is the role of the condensation product H2O which stabilizes the product via hydrogenbonding. This study thus highlights several key aspects of computational investigations towards the selective inhibitor layer buildup of SMIs: thermodynamic driving force and kinetic preference for SMI on NGS, role of reactive surface groups, surface chemistry steps before and after the ratedetermining transition state. Thus, it transpires that for an understanding of selectivity in SMIbased approaches, a detailed analysis of the surface chemistry is crucial.