Water clusters confined in icosahedral fullerene cavities

Abstract

Likely candidates for the global energy minima of endohedral (H2O)N@C60 and (H2O)N@C180, and exohedral (H2O)NC180 water–fullerene clusters with N⩽20, are found using basin-hopping global optimization. The potential energy surfaces are constructed using both the rigid TIP4P and the flexible q-TIP4P/F potentials to model the water–water interaction, together with a Lennard–Jones potential for the water–fullerene interaction. In agreement with previous ab initio studies, we find that the small C60 cavity is able to encapsulate exothermically only one water molecule. On the other hand, the larger C180 cavity can encapsulate up to 17 water molecules exothermically. This threshold value is higher than that reported in a previous ab initio study (N⩽12). New confined water cluster structures are found. One which is particularly interesting is the structure of (H2O)14@C180, with the water molecules forming an internal cage in which six oxygen atoms are located at the vertices of an almost regular octahedron and the eight remaining ones lie on top of the octahedron faces. For N⩾15 one water molecule is always present at the center of the water cage, which is distorted to accommodate the extra molecules.