Here, the electronic energy (EQM) based on an ab initio calculation is one major contribution to the enthalpy. In practice these terms are often formulated as corrections to the inner energy U. In these cases, enthalpy is derived based on 𝐻 = 𝑈 + 𝑘 B 𝑇. Within the RRHO approximation, every component of eq. 1.3 and eq. 1.4 is formulated based on its partition function and the ideal gas law. In consequence, only the harmonic vibrational frequencies of the system and EQM have to be calculated by ab initio methods. With more complex approaches even anharmonic corrections to the vibrational terms can be included. However, due to the computational expense of this method this is mostly unfeasible for surface chemistry questions. The reaction barrier is the second major factor determining the ALD reactivity and even more so the selectivity in AS-ALD. If transition state theory (TST) is assumed to be applicable, the reaction rate k of the chemical reactions in the ALD process can be determined directly from the reaction barrier ΔG ‡ via the Eyring equation (eq. 1.5). The prefactor A can then either be determined from experience (or assumed to be the same for all competing reactions) or it can be computed via TST (qq-TST). There are possibilities to move beyond the TST picture by capturing effects like reaction probability, but they are usually out of scope for investigations of ALD chemistry.