The atomic and electronic structure of solids is best described with the help of a unit cell and PBC. This approach can be adapted to AS-ALD by modelling the surface as a slab consisting of several layers in the unit cell using PBC (Fig. .). For technical reasons (the plane-wave basis sets typically used require 3D PBC), the slab is repeated in the third dimension with a vacuum layer to separate it from the next periodic image. Also in this approach, several parameters have to be chosen appropriately for the results to be meaningful: number of layers in the slab, vacuum thickness, frozen layers, etc. The surface model is of utmost importance: setting up the unit cell and deciding about the surface termination requires experimental knowledge of the substrate under AS-ALD conditions. This is well-known for some surfaces (e.g., Si(001)) but much less clear for amorphous (e.g., SiO2, Al2O3) or complex substrates. In this case, setting up a good surface model requires in-depth investigations. Also, the change in surface termination during the AS-ALD process is rarely known. Considering the periodicity of the surface has the advantage of delivering an accurate description of the electronic structure and avoiding finite-size effects. Different coverages can be simulated on an equal footing, and structures and energies are less dependent on the model chosen (if the parameters are well-converged). The main drawbacks are the considerably larger computational cost (10-100x) compared to molecular cluster computations and the lack of accurate methods beyond DFT. In conclusion, slab computations are the preferred choice for AS-ALD modeling.