Global potential energy minima of C60(H2O)n clusters

Likely candidates for the global potential energy minima of C-60(H2O)(n) clusters with n <= 21 are found using basin-hopping global optimization. The potential energy surfaces are constructed using the TIP4P intermolecular potential for the water molecules, a Lennard-Jones water-fullerene potential, and a water-fullerene polarization potential, which depends on the first few nonvanishing C-60 multipole polarizabilities. This combination produces a rather hydrophobic water-fullerene interaction. As a consequence, the water component of the lowest C-60(H2O) n minima is quite closely related to low-lying minima of the corresponding TIP4P (H2O) n clusters. In most cases, the geometrical substructure of the water molecules in the C-60(H2O) n global minimum coincides with that of the corresponding free water cluster. Exceptions occur when the interaction with C-60 induces a change in geometry. This qualitative picture does not change significantly if we use the TIP3P model for the water-water interaction. Structures such as C-60@(H2O)(60), in which the water molecules surround the C-60 fullerene, correspond to local minima with much higher potential energies. For such a structure to become the global minimum, the magnitude of the water-fullerene interaction must be increased to an unphysical value.