Positron binding in large water clusters: insights into cage-structured systems

We present a computational quantum chemical study of positron binding to cage-structured large water clusters (H₂O)_<i>n</i> with selected sizes in the range from <i>n</i> = 8 to 36. Positron binding energies for various conformations of each cluster size were calculated using the density functional theory-based electron-positron correlation-polarization potential method. We obtained that the positron binding energies of the lowest energy structures of water clusters overall tend to increase with the number of water molecules. Furthermore, we found the positronic first- and second excited states for clathrate structures for <i>n</i> ≥ 20. These calculations revealed that positron binding exhibits both internally localized character in the ground state and surface-localized character in the excited sates, respectively. The energetic properties and behaviors of the energetically low-lying interior-bound positron states exhibit trends analogous with those well known for excess electrons in the water cluster anions. The systematic size-dependent trend of positron binding energies allows us to quantify its limiting value in the limit of an infinite number of water molecules, associated with hydrated positron states.