MOLECULAR PHYSICS, 1992, VOL. 75, NO. 3, 633-651 An extended RISM study of simple electrolytes: pair correlations in a NaCI-SPC water model By GERHARD HUMMERt and DIKEOS M. SOUMPASIS Abteilung Molekulare Biologic, Max-Planck-Institut f/Jr Biophysikalische Chemic, Postfach 2841, D-3400 G6ttingen, Germany (Received 3 May 1991; accepted 2 August 1991 ) We have used the extended RISM approach in conjunction with the SPC model of water to compute the ten-pair correlation functions of aqueous NaC1 systems in a wide range of salt concentrations (0-5 M). The numerical results are tested against experimental neutron scattering data and computer simulations wherever the latter are available and/or feasible. Overall the positions of the structural features in the correlation functions are reproduced quite well by the computation but other details such as some peak heights and the ion water coordination numbers obtained are less satisfactory. We discuss several problems encountered due to the assumptions inherent in the extended R1SM approach one of which is the incorrect dielectric screening behaviour of this theory. 1. Introduction Over the last few years it has been shown [1-5] that it is possible to describe rather subtle ionic effects on charged biomolecular structures (e.g. salt induced DNA struc- tural transitions [6]) using a potentials of mean force (PMF) framework based on the theory of homogeneous electrolytes. In the simplest implementation of the PMF approach currently used in routine investigations of biophysical interest, we model 1 : 1 electrolytes via the standard restricted primitive model (RPM). Although this suffices to obtain results and predictions of surprising accuracy in many applications, we currently investigate a number of possibilities to construct refined PMF implemen- tations, where the solvent (water) is more realistically modelled, i.e. beyond the RPM dielectric continuum picture. This proves to be an entirely non-trivial task due to our still incomplete knowledge of aqueous electrolytes both at the experimental and the theoretical level. The situ- ation can briefly be summarized as follows. Thanks to many advances both in X-ray scattering (review in [7, 8]) and especially the isotopic substitution differential neutron scattering techniques pioneered by Enderby and co-workers (reviewed in [8-10]) a rather clear picture of ionic hydration in concentrated solutions has emerged, but unfortunately it is still not possible to determine ion-ion correlations for most electrolytes and concentration regimes ( ~< 5 M) of interest to us. For instance in alkali halide systems the C1-C1 pair correlation function is known only in the case of a 14.90 m (molal) LiC1 [11] and no results for ion-cation correlations are available yet. This is also the case for ion-water correlation functions in the concentration regime below about 1 M (molar). Following the pioneering ab initio computations of ion-water [12-14] and water- 5" Also: Institut fiir Experimentalphysik, Universit~t Wien, Strudlhofgasse 4, A-1090 Wien, Austria. 0026-8976/92 $3.00 9 1992 Taylor & Francis Ltd