Selectivity in AS-ALD stems from the difference in precursor reactivity with GS and NGS, either via an intrinsic reactivity difference or after modification of one surface (activation or inhibition). Thus, the state of the surface is crucial for this selectivity and has been the target of computational modeling. In ultra-high vacuum (UHV) surface science investigations in the last decades, several reliable surface models were derived and used in computational modeling. This is especially true for elemental semiconductors, low-index metal surfaces and binary oxide surfaces. For more complex or less-investigated substrates, it is often necessary to develop a new surface model. But even in the case of established models, the lack of detailed information from the experimental studies on the exact nature of the substrate often impedes an accurate representation. Hence, setting up a suitable model for the surface in AS-ALD is a major challenge and crucially determines the accuracy of the results compared to experiment. The modelling would highly benefit from more studies revealing detailed experimental information about the surface state under ALD condition which is commonly done with UHV surface science approaches. While some promising new developments exist, this information is often scarce and leaves some ambiguity in setting up the computational surface model.