Examination of the hydrogen-bonding networks in small water clusters (n = 2-5, 13, 17) using absolutely localized molecular orbital energy decomposition analysis.

Using the ωB97X-D and B3LYP density functionals, the absolutely localized molecular orbital energy decomposition method (ALMO-EDA) is applied to the water dimer through pentamer, 13-mer and 17-mer clusters. Two-body, three-body, and total interaction energies are decomposed into their component energy terms: frozen density interaction energy, polarization energy, and charge transfer energy. Charge transfer, polarization, and frozen orbital interaction energies are all found to be significant contributors to the two-body and total interaction energies; the three-body interaction energies are dominated by polarization. Each component energy term for the two-body interactions is highly dependent on the associated hydrogen bond distance. The favorability of the three-body terms associated with the 13- and 17-mer structures depends on the hydrogen-donor or hydrogen-acceptor roles played by each of the three component waters. Only small errors arise from neglect of three-body interactions without two adjacent water molecules, or beyond three-body interactions. Interesting linear correlations are identified between the contributions of charge-transfer and polarization terms to the two and three-body interactions, which permits elimination of explicit calculation of charge transfer to a good approximation.