  The many-body polarization energy is the major source of non-additivity in
strongly polar systems such as water. This non-additivity is often considerable
and must be included, if only in an average manner, to correctly describe the
physical properties of the system. Models for the polarization energy are
usually parameterized using experimental data, or theoretical estimates of the
many-body effects. Here we show how many-body polarization models can be
developed for water complexes using data for the monomer and dimer only using
ideas recently developed in the field of intermolecular perturbation theory and
state-of-the-art approaches for calculating distributed molecular properties
based on the iterated stockholder atoms (ISA) algorithm. We show how these
models can be calculated, and validate their accuracy in describing the
many-body non-additive energies of a range of water clusters. We further
investigate their sensitivity to the details of the polarization damping models
used. We show how our very best polarization models yield many-body energies
that agree with those computed with coupled-cluster methods, but at a fraction
of the computational cost.
