Water hydrogen degrees of freedom and the hydrophobic effect.

Hydrogen bonds are the key interaction that establishes the liquid and solvent properties of water. Nevertheless, it is possible to construct an accurate molecular model of water which does not include hydrogens or any orientational interactions. Using this model, we calculate the structural and thermodynamic properties for the hydration of methane and ethane. The addition of the hydrophobic solute leads to changes in structure, as can be seen in slightly enhanced tetrahedral geometries and slightly reduced Voronoi volumes of water near the solute. The entropy of hydration from the model is about half the experimental value, suggesting that what is left out of the model-the orientational or hydrogen response-contributes to about half the entropy. For the hydrophobic association of two methane molecules in water, the hydrogen degrees of freedom do not seem to play an important role and the entropy of association is similar to all-atom models.