4 FIG. 5: (Color online) RDF obtained from MC simulations (diamond), BIMSA3 (solid line), and MSA-fit (dot dashed) at two concentrations. r (Å) 2 4 6 8 4 6 8 0.5 1 1.5 4 6 8 0.5 mol.L -1 g$_{12}$(r) 1.5 mol.L -1 g$_{12}$(r) 1.5 mol.L -1 g$_{11}$(r) 1.5 mol.L -1 g$_{22}$(r) The RDF obtained within BIMSA3 are compared with the MC and MSA-fit results in Fig. 5. Our BIMSA3 model accounts for the strong molecular peak of the CIP and provides the correct distances of minimal approach; whereas the naive MSA-fit procedure ignores the former and gives poor estimates for the latter. At larger sep- arations, the BIMSA3 results do not reproduce the os- cillations observed in the MC simulations, but the cor- responding energy oscillations in the effective potentials are less than k$_{B}$T . In addition, the perturbation term of the BIMSA3 appears to be negligible compared to the $_{reference term for concentrations less than 1 mol l}$− $^{1}$. The perturbation can then be omitted to obtain a fully ana- lytical theory, determined by the hard sphere diameters and the pair fraction given by LPT; with the free energy and the RDF given in terms of the BIMSA and MSA so- lutions, as described above. While the procedure we have followed uses two different approximations for the refer- ence and perturbation terms (MSA vs BIMSA), these are known to be accurate for the systems under consideration and do not appear to be inconsistent with each other. To conclude, we have combined MD simulations with LPT to construct simple models of electrolyte solutions which account for the molecular nature of the solvent. The final result is fully analytical and it yields the ther- modynamic and structural properties of the solution, in agreement with the original molecular description. The methodology can in principle be adapted to any molecu- lar description of the system (MD simulations involving interaction potentials accounting for polarization effects or Car-Parrinello MD simulations for example) as long as the ion-ion RDF are known. It can also be generalized to study interfaces. The method appears to be a promis- ing approach toward the description of the specific effects of ions, especially for complex systems whose modeling requires an analytic solution. The authors are particularly grateful to Werner Kunz for fruitful discussions. [1] W. G. McMillan and J. E. Mayer, J. Chem. Phys. 13 , 276 (1945). [2] J. M. G. Barthel, H. Krienke, and W. Kunz, Physical Chemistry of Electrolyte Solutions (Springer, 1998). [3] L. Blum, in Theoretical Chemistry: Advances and Per- spectives , edited by H. Eyring and D. Henderson (Aca- demic Press, 1980), vol. 5, pp. 1–66. [4] L. Blum and O. Bernard, J. Stat. Phys. 79 , 569 (1995). [5] J.-F. Dufrˆeche et al., J. Phys. Chem. B 109 , 9873 (2005). [6] P. Jungwirth and D. J. Tobias, Chem. Rev. 106 , 1259 (2006). [7] W. Kunz, P. LoNostro, and B. W. Ninham, Curr. Opin. Colloid Interface Sci. 9 , 1 (2004). [8] B. Hess, C. Holm, and N. van der Vegt, Phys. Rev. Lett. 96 , 147801 (2006). [9] I. Kalcher and J. Dzubiella, J. Chem. Phys. 130 , 134507 (2009). [10] S. Gavryushov and P. Linse, J. Phys. Chem. B 110 , 10878 (2006) [11] A. P. Lyubartsev and A. Laaksonen, Phys. Rev. E 52 , 3730 (1995). [12] D. Horinek and R. R. Netz, Phys. Rev. Lett. 99 , 226104 (2007). [13] M. Lund, P. Jungwirth, and C. E. Woodward, Phys. Rev. Lett. 100 , 258105 (2008). [14] S. Van Damme et al., J. Phys. Chem. B 113 , 3105 (2009). [15] J.-P. Hansen and I. R. McDonald, Theory of Simple Liq- uids (Academic Press, 1986). [16] J. C. Rasaiah and R. M. Lynden-Bell, Philos. Trans. R. Soc. London, Ser. A 359 , 1545 (2001). [17] A. P. Lyubartsev and S. Marcelja, Phys. Rev. E 65 , 041202 (2002). [18] V. M. M. Lobo, Electrolyte Solutions, Data on Thermo- dynamic and Transport Properties , vol. I-II (Coimbra Ed- itora, Lisbon, Portugal, 1984). [19] G. Ciccotti, P. Turq, and F. Lantelme, Chem. Phys. 88 , 333 (1984). [20] J.-F. Dufrˆeche, T. O. White, and J.-P. Hansen, Mol. Phys. 101 , 1741 (2003). [21] The average contact distance between a symmetric dumbbell and an infinite plane at β = 0.