  The adsorption and dissociation of isolated water molecule on Zr(0001)
surface are theoretically investigated for the first time by using
density-functional theory calculations. Two kinds of adsorption configurations
with almost the same adsorption energy are identified as the locally stable
states, i.e., the flat and upright configurations respectively. It is shown
that the flat adsorption states on the top site are dominated by the
1$b_{1}$-$d$ band coupling, insensitive to the azimuthal orientation. The
diffusion between adjacent top sites reveals that the water molecule is very
mobile on the surface. For the upright configuration, we find that besides the
contribution of the molecular orbitals 1$b_{1}$ and 3$a_{1}$, the
surface$\rightarrow$water charge transfer occurring across the Fermi level also
plays an important role. The dissociation of H$_{2}$O is found to be very
facile, especially for the upright configuration, in good accordance with the
attainable experimental results. The present results afford to provide a
guiding line for deeply understanding the water-induced surface corrosion of
zirconium.
