Al2O3 (Fig. .c). By a combination of DFT and RSA approaches, they find that on the one hand HAc benefits from its smaller size which leads to denser packed SMI layers via chemical passivation (Fig. .b). Hthd on the other hand benefits from its bulkiness leading to better steric blocking (Fig. .a). Finally, the authors compiled these insights into the conclusion that a good SMI either should target high chemical passivation or steric blocking. This leads to better selectivity than a precursor with a medium performance in both mechanisms. Further studies will be required to confirm this as a general conclusion, especially, since other studies conclude that a combination of steric blocking and chemical passivation should be targeted. This discrepancy is mainly a reflection of the lack of reliable data on the question of blocking mechanisms and hopefully more studies will appear addressing this in the future. An important parameter for the question of chemical passivation is the density of reactive groups at the surface which is rarely investigated experimentally. This information is crucial for setting up suitable computational models correctly representing the experimentally observed reactive group density correctly (discussed in more detail in section 3.2). In combination with the knowledge of the theoretical coverage limit of an SMI, this gives an idea of how many reactive surface groups are left in the high-coverage limit. The precursor can play a major role in the effectiveness of the blocking mechanism, especially w.r.t. decomposition reactions of the SMI layer and penetration which are discussed in the next section.