Global potential energy minima of (H2O)n clusters on graphite

Likely candidates for the global potential energy minima of (H2O)(n) clusters with n <= 21 on the (0001) surface of graphite are found using basin-hopping global optimization. The potential energy surfaces are constructed using the TIP4P intermolecular potentials for the water molecules (the TIP3P is also explored as a secondary choice), a Lennard-Jones water-graphite potential, and a water-graphite polarization potential that is built from classical electrostatic image methods and takes into account both the perpendicular and the parallel electric polarizations of graphite. This potential energy surface produces a rather hydrophobic water-graphite interaction. As a consequence, the water component of the lowest graphite-(H2O)(n) minima is quite closely related to low-lying minima of the corresponding TIP4P (H2O)(n) clusters. In about half of the cases, the geometrical substructure of the water molecules in the graphite-(H2O)(n) global minimum coincides with that of the corresponding free water cluster. Exceptions occur when the interaction with graphite induces a change in geometry. A comparison of our results with available theoretical and experimental data is performed.