Balancing Local Order and Long-Ranged Interactions in the Molecular Theory of Liquid Water

A molecular theory of liquid water is identified and studied on the basis of computer simulation of the TIP3P model of liquid water. This theory would be exact for models of liquid water in which the intermolecular interactions vanish outside a finite spatial range, and therefore provides a precise analysis tool for investigating the effects of longer-ranged intermolecular interactions. We show how local order can be introduced through quasi-chemical theory. Long-ranged interactions are characterized generally by a conditional distribution of binding energies, and this formulation is interpreted as a regularization of the primitive statistical thermodynamic problem. These binding-energy distributions for liquid water are observed to be unimodal. The gaussian approximation proposed is remarkably successful in predicting the Gibbs free energy and the molar entropy of liquid water, as judged by comparison with numerically exact results. The remaining discrepancies are subtle quantitative problems that do have significant consequences for the thermodynamic properties that distinguish water from many other liquids. The basic subtlety of liquid water is found then in the competition of several effects which must be quantitatively balanced for realistic results.Comment: 8 pages, 6 figure

Self Consistent Molecular Field Theory for Packing in Classical Liquids

Building on a quasi-chemical formulation of solution theory, this paper proposes a self consistent molecular field theory for packing problems in classical liquids, and tests the theoretical predictions for the excess chemical potential of the hard sphere fluid. Results are given for the self consistent molecular fields obtained, and for the probabilities of occupancy of a molecular observation volume. For this system, the excess chemical potential predicted is as accurate as the most accurate prior theories, particularly the scaled particle (Percus-Yevick compressibility) theory. It is argued that the present approach is particularly simple, and should provide a basis for a molecular-scale description of more complex solutions.Comment: 6 pages and 5 figure

Hydrophobic interactions: conformational equilibria and the association of non-polar molecules in water

Recently developed proximity approximations have been used to calculate inhomogeneous water density profiles around non-polar molecular solutes. Relative Helmholtz energies of hydrophobic hydration are calculated from these density profiles using two inherently different approaches: Helmholtz energy perturbation and a multiparticle correlation function expansion. Entropic contributions to the hydration Helmholtz energy are also calculated using the multiparticle correlation function expansion for the entropy truncated at the level of pair correlations. We show that the proximity approximations describe water structure around a tetramethylammonium ion in good agreement with neutron diffraction experiments, and provide an accurate description of water structure around simple alkanes and benzene as reflected in their entropies of hydration. Further, we reproduce two important features of hydrophobic interactions: a highly favoured contact minimum and a solvent separated minimum in the PMFs for methane–methane and neopentane–neopentane association in water. Our calculations also show that the more compact conformations of n-butane and n-pentane are favoured in water, as expected based on traditional ideas regarding hydrophobic interactions