Effect of hydrogen bonds on polarizability of a water molecule in (H₂O)_N (N = 6, 10, 20) isomers

Polarizabilities of the low-lying isomers of (H₂O)_N (N = 6, 10, 20) clusters were computed by using Density Functional Theory. The global polarizabilities of the water isomers were found to depend mainly on the total number of water molecules rather than their cluster structures. We show that this result hides in fact a strong heterogeneity of the molecular polarizability within the different isomers. The global polarizability of a cluster was divided into a sum of molecular contributions by using the Hirshfeld partitioning scheme. We reveal that the value of the local polarizability of a molecule in the cluster is correlated with the number and type of the hydrogen bonds (HB) the molecule forms. Consequently, the molecules located in the interior of the cluster, which usually form more HBs, have smaller molecular polarizabilities than the molecules at the surface, which form less HBs. The contribution of intermolecular interaction to the global polarizability was analyzed by decomposing the cluster polarizability into intra- and inter-molecular contributions. The former measures the polarization within the molecular basin against the external electric field, while the latter is described as the sum of polarizability caused by charge flow through the HBs. These two contributions vary with the cluster size: the intermolecular contribution decreases with the cluster size on the contrary of the intramolecular contribution which increases.