Many-Body Convergence of the Electrostatic Properties of Water.

The many-body convergence of the dipole moment and the dipole-dipole polarizability of water is investigated. It is found that, for systems of low symmetry like the water clusters examined here, simple measures such as dipole magnitudes and average polarizabilities may lead to an incomplete interpretation of the underlying physics. Alternative metrics are introduced that allow for an unambiguous characterization of both properties. The convergence of the many-body decomposition of the total dipole and the polarizability is studied for (H2O)N, with N = 4-6 being minimum energy water clusters and N = 14 being clusters that were extracted from condensed phase simulations. For these clusters, it is demonstrated that both the total dipole and polarizability are almost entirely pairwise additive, with three-body terms contributing less than 4% and all higher-order terms being essentially negligible.