So far, we have only considered 2-dimensional processes. However, to get a complete picture of surface growth, 3-dimensional processes such as upper-step diffusion followed by its descent are particularly important. The activation energies of the respective processes (stepdown, step-down (dimer), upper-step) as well as the calculated Ehrlich-Schwoebel barriers for Mg(0001) and Mg(101 ! 1) are summarized in Table and Table XIV. The schematic illustrations of the processes are given in Figure and Figure . Ehrlich-Schwoebel barriers indicating a uniform and smooth growth due to a high rate of interlayer mass transport. Likewise, the descent is clearly preferred to the ascent following the observed increase in coordination partners. All step-down barriers are in agreement with the literature. Going one step further, we wanted to investigate the influence of a second adatom on the step-down barriers. It becomes immediately apparent that the barriers to descent for hopping [ED0/2 ↔ ED1/3, Ea for = 0.58/0.67 eV, Ea rev = 0.74/0.68 eV] as well as exchange processes [ED0/2 ↔ ED1/3 (Ex.), Ea for = 0.21/0.11 eV, Ea rev = 0.38/0.15 eV] increase significantly. It can be assumed that the larger the cluster, the stronger the effect. For smooth surface growth, the adatom must reach the step-edge faster than it reaches another adatom. Thus, the higher the deposition rate, the more likely new islands aggregate on top of existing islands before the bottom layer is fully complete. A further interesting case is the exchange process at the [1 ! 1 ! 20]-step. In this process, the barriers for ascent and descent are very close together. However, it is important to note that the second atom follows the diffusing atom into the fcc position above the exchange atom during descent. This arrangement maintains the dimer conformation until beyond the TS (SI Figure ) and lowers the activation energy of the process to half of the corresponding exchange process at the [112 ! 0]-step. Ex.