Aqueous interaction site integral-equation theory that exactly takes into account intramolecular correlations

We report the development of a formally exact integral equation for the three-dimensional hydration structure around molecular solutes of arbitrary complexity. A distinctive feature of our theory--termed aqueous interaction site (AXIS) integral-equation theory--is that it fully takes into account the intramolecular structural correlations of solvent water, which has been missing in the previous integral-equation theories such as the three-dimensional reference interaction site model (3D-RISM) theory. With a simplifying approximation in which the intermolecular bridge function is neglected, an illustrative application of the AXIS theory is made on the equilibrium oxygen and hydrogen distributions of solvent water surrounding a solute water molecule at ambient and supercritical conditions. We demonstrate through a comparison with molecular dynamics simulation results that the inclusion of the exact intramolecular correlations improves upon the 3D-RISM theory in describing the water distribution around molecular solute, in particular near the surface region of the solute molecule, though there still remain quantitative differences from the simulation results. To further improve the quantitative accuracy of the theory, one needs to incorporate the intermolecular bridge function, and a possible formulation for the approximate bridge function is suggested based on the angular decomposition.