Size-dependent transition from all-surface to interior-molecule structures in pure neutral water clusters

Global geometry optimizations of pure neutral water clusters in the size range n = 2-30 have been performed, using the TTM2-F potential and a massively parallel implementation of an evolutionary algorithm. A size-dependent transition from all-surface to water-centered structures is found, starting at n = 17. Global minimum structures for n = 13,17,21,22,... have qualitatively different arrangements of water molecules from those found previously for the much simpler TIP4P potential, indicating an inability of this potential to model this structural transition. Which features of these two potentials are responsible for the structural differences observed is investigated. Simulations of infrared spectra in the OH-stretch region indicate that it may be possible to differentiate experimentally between the all-surface and water-centered structures found here.