  We demonstrate for the first time a tight binding model for water
incorporating polarizable anions. A novel aspect is that we adopt a "ground up"
approach in that properties of the monomer and dimer only are fitted.
Subsequently we make predictions of the structure and properties of hexamer
clusters, ice-XI and liquid water. A particular feature, missing in current
tight binding and semiempirical hamiltonians, is that we reproduce the almost
two-fold increase in molecular dipole moment as clusters are built up towards
the limit of bulk liquid. We concentrate on properties of liquid water which
are very well rendered in comparison with experiment and published density
functional calculations. Finally we comment on the question of the contrasting
densities of water and ice which is central to an understanding of the
subtleties of the hydrogen bond.
