The challenges are associated with the many detailed decisions necessary to set up the cluster model correctly. Cluster size, partial freezing of edge atoms, saturating atoms at the edges (which atoms, which bond lengths), how to deal with the frozen atoms in thermochemistry calculations are just some of the decisions to be made. Yet, the major drawback is its neglect of the surface periodicity. First, it cannot capture the full electronic structure of the solid (molecular orbitals instead of band structure) which is especially relevant for conducting surfaces (for which the approach is nowadays hardly used anymore). Second, while structures are often quite reliable, energies can strongly depend on the size of the cluster and usually deviate from slab results. Especially reaction barriers are often found to be very high compared to extended slab modelling. Third, the possible reactions with the surface are limited to the atoms included in the cluster and pathways might be missed which need neighboring atoms (an example is the reaction of benzylazide on Si(001) ). Lastly, the investigation of coverage-dependent reactions is not possible. However, in studies by Chabal et al., frequently a good agreement between cluster and slab approaches was found for precursor adsorption on differently terminated Si(001) surfaces. The cluster approach is thus suitable for efficient screening of surface chemistry but not the method of choice for accurate in-depth investigations of AS-ALD.