  We present a comprehensive density-functional theory study addressing the
adsorption, dissociation and successive diffusion of water molecules on the two
regular terminations of SrTiO3(001). Combining the obtained supercell-geometry
converged energetics within a first-principles thermodynamics framework we are
able to reproduce the experimentally observed hydroxilation of the
SrO-termination already at lowest background humidity, whereas the
TiO2-termination stays free of water molecules in the regime of low water
partial pressures. This different behavior is traced back to the effortless
formation of energetically very favorable hydroxyl-pairs on the prior
termination. Contrary to the prevalent understanding our calculations indicate
that at low coverages also the less water-affine TiO2-termination can readily
decompose water, with the often described molecular state only stabilized
towards higher coverages.
