As stated above, ALD happens in a reactor. Since time scales of seconds and length scales of centimeters are unavailable by atomistic modeling this cannot directly be modeled. However, as outlined in the introduction, gas flow might be an important factor in understanding certain AS-ALD processes. We briefly outline major approaches here. CFD approaches uses numerical methods to solve macroscopic equations of fluid dynamics, and this is also applicable to gas flow in 3D. The nonlinearity of gas flow yields quite complex equations, making HPC resources indispensable. Moreover, interaction with the surface can be incorporated. The results are timedependent concentration of gas components which can then be used as input for microscopic modelling. FE methods are used to numerically solve a set of coupled differential equations. The system is decomposed into several geometrically simple sub-systems (finite elements) which can then be solved efficiently. Multiscale modeling refers to methods combining several computational approaches suitable for different time and length scales. Exemplified for AS-ALD, thermodynamics properties and reaction barriers from DFT can be utilized as input for kMC simulations, whose product distribution can feed a CFD reactor model. The Christofides group applies this approach to ALD, and recently also to AS-ALD. Here, the coupling of the "scales" is the crucial aspect since information from one scale is not trivially included as information in the next scale. A second challenge is the coupling from large scales back to the microscopic scales, i.e., the results from flow analysis on CFD level back to the DFT computation. Other communities, e.g., theoretical heterogeneous catalysis, have been working on this problem for decades.