  Using first-principles calculations, we systematically study the adsorption
behavior of a single molecular H$_{2}$O on the Be(0001) surface. We find that
the favored molecular adsorption site is the top site with an adsorption energy
of about 0.3 eV, together with the detailed electronic structure analysis,
suggesting a weak binding strength of the H$_{2}$O/Be(0001) surface. The
adsorption interaction is mainly contributed by the overlapping between the $s$
and $p_{z}$ states of the top-layer Be atom and the molecular orbitals 1$b_{1}$
and 3$a_{1}$ of H$_{2}$O. The activation energy for H$_{2}$O diffusion on the
surface is about 0.3 eV. Meanwhile, our study indicates that no dissociation
state exists for the H$_{2}$O/Be(0001) surface.
