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Likely candidates for the global potential energy minima of C60(H2O)n clusters with n < or = 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 C60 multipole polarizabilities. This combination produces a rather hydrophobic water-fullerene interaction. As a consequence, the water component of the lowest C60(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 C60(H2O)n global minimum coincides with that of the corresponding free water cluster. Exceptions occur when the interaction with C60 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 C60@(H2O)60, in which the water molecules surround the C60 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.

The effects of confinement on water clusters inside nonmetallic carbon nanotubes with radii ranging between 4 and 7.5 Å have been computationally investigated by means of global optimization and finite temperature simulations. The water−water interaction is described by the TIP4P rigid body potential, and a Lennard-Jones potential is used for the water− carbon interaction. Water clusters containing up to 20 molecules are found to form 1D chainlike configurations for the narrow (7, 5) nanotube and 2D ladderlike structures in the (7, 6) tube. In wider tubes, 3D configurations are then formed showing helical motifs, ringlike or closed cage structures, before the most stable structure on flat graphene is eventually found. The same results are obtained by replacing the fully atomistic water−nanotube potential by its continuous approximation [Bretoń, J.; Gonzaĺez-Platas, J.; Giradet, C. J. Chem. Phys. 1994, 101, 3334], indicating a negligible effect of corrugation. The effects of additional nanotubes were also considered with the adsorption energies being found to converge rather quickly already for the triple-wall tube. Parallel tempering Monte Carlo simulations of the water octamer reveal a counterintuitive decrease in the melting point relative to the free-standing case. Molecular dynamics simulations show that melting is concomitant with some axial diffusion of the water molecules, and with radial diffusion perpendicular to the tube axis remaining limited. In accordance with previous studies concerned with bulk water, the weakening of the cluster thermal stability is interpreted as being caused by the hydrophobic character of the carbon−water interaction.

The microcanonical analysis is shown to be a powerful tool to characterize the protein folding transition and to neatly distinguish between good and bad folders. An off-lattice model with parameter chosen to represent polymers of these two types is used to illustrate this approach. Both canonical and microcanonical ensembles are employed. The required calculations were performed using parallel tempering Monte Carlo simulations. The most revealing features of the folding transition are related to its first-order-like character, namely, the S-bend pattern in the caloric curve, which gives rise to negative microcanonical specific heats, and the bimodality of the energy distribution function at the transition temperatures. Models for a good folder are shown to be quite robust against perturbations in the interaction potential parameters.

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.