On Mg(101 ! 1) (Figure , Table ), however, there are clearly distinguishable barriers for forward and reverse processes. In the case of two atoms approaching each other within a channel, the forward barrier is about half as large as the reverse barrier [d0 ↔ d1, Ea for = 0. If a third atom is approaching, a linear or a triangular trimer is formed, depending on the side of the attack. For Mg(0001), no or only extremely low barriers could be determined [T0 → T1, Ea for = 0.00 eV; TL0/1 → TL2, Ea for = 0.01/0.02 eV], as was already the case for the dimer formation. In turn, the linear form can transform itself into the thermodynamically more stable triangular trimer form. However, from a kinetic point of view, the barrier of the interchange to a hcp trimer is slightly lower [TI0 → TI1, Ea = 0.06 eV] than that of the fcc counterpart [TI0 →