A 3-DIMENSIONAL REDUCTION OF THE ORNSTEIN-ZERNICKE EQUATION FOR MOLECULAR LIQUIDS

The derivation of a three-dimensional integral equation for solute mol ecule-solvent site correlation functions is presented. The equation is obtained by averaging the Ornstein-Zernicke equation for molecular li quids over orientations of the solvent molecule consistent with one si te of the solvent remaining at a fixed distance from a solute-based or igin. The approach is similar to that adopted in the reduction leading to the reference interaction site model (RISM) equations but retains full three-dimensional information regarding the structure of the refe rence solute molecule. The proposed equation can be solved using three -dimensional HNC-like closures, of which three different forms are dis cussed. A formulation which allows the introduction of long range inte ractions through a renormalization of the equation is also presented. Applications to various molecular liquids indicate that the proposed t heory provides pair correlation functions that are in better agreement with molecular dynamics simulations than those obtained using the ext ended RISM formulation. Furthermore, qualitative errors in the correla tion functions, frequently seen in results from RISM calculations are completely eliminated through geometrical averaging of the Mayer funct ion in the 3D HNC closure. Prospects for the development of a novel me an field theory of solvation are also discussed. (C) 1997 American Ins titute of Physics.