  Diffusion barriers for a cluster of three water molecules on Pd(111) have
been estimated from ab-initio Density Functional Theory. A model for the
diffusion of the trimer based in rotations yields a simple explanation of why
the cluster can diffuse faster than a single water molecule by a factor
$\approx 10^{2}$. This model is based on the differences between the adsorption
geometry for the three monomers forming the cluster. One member interacts
strongly with the surface and sits closer to the surface (d) while the other
two interact weakly and stay at a larger separation from the surface (u). The
trimer rotates nearly freely around the axis determined by the d monomer.
Translations of the whole trimer imply breaking the strong interaction of the d
monomer with the surface. Alternatively, thermal fluctuations exchange the
actual monomer sitting closer to the surface with a lower energetic cost.
Rotations around different axis introduce a diffusion mechanism where a strong
interaction is kept along the diffusion path between the water molecule
defining the axis of rotation and the Pd underneath.
