Classical density functional calculation of radial distribution functions of liquid water

- Classical density functional theory for liquid water is formulated.
- Triplet correlations and bridge functions are taken into account.
- Factorization approximation for ternary direct correlation functions is employed.
- Radial distribution functions for liquid water are calculated and compared with simulation results.

We have calculated the radial distribution functions (RDFs) of liquid water on the basis of the classical density functional theory combined with the reference interaction site model for molecular liquids. The density expansion for the Helmholtz free energy functional is retained up to the third order in order to take into account the effects of the bridge functions beyond the hypernetted-chain (HNC) approximation. The ternary direct correlation functions in the expression of the bridge functions are then given by a factorization approximation in terms of the site-site pair correlation functions, thus leading to a closed set of integral equations for the determination of the RDFs. We have obtained a numerical result in which a poor description by the HNC approximation for the second peak of the oxygen-oxygen RDF at room temperature has been improved to some extent by incorporating the oxygen-oxygen bridge function. Some directions toward more satisfactory agreement with computer simulation results are addressed as well.