text: We ﬁrst used LPT for a two-component system (Na+ 
  and Cl− free ions) within the MSA (model MSA2), for 
  1. The mini- 
  concentrations ranging from 0.1 to 2.0 mol l− 
  mization leads to almost constant diameters on the whole 
  range of concentration: σ1 = 3.67 ˚A and σ2 = 4.78 ˚A. 
  As shown in Fig. 2, these parameters yield osmotic co- 
  eﬃcients close to MC calculations only at very low con- 
  1 (experimental values are 
  centration, i.e., c ≤ 0.1 mol l− 
  given for indicative purposes only, since a perfect model 
  will exactly match the MC results). For molar solutions, 
  the LPT results diﬀer considerably from MC calculations. 
  This discrepancy can easily be understood by comparing 
  the diameters found within the MSA2 calculation with 
  the eﬀective potentials given in Fig. 1. The anion/cation 
  contact distance obtained within the MSA2 calculation 
  is 4.2 ˚A, which is in the region of the second minimum of 
  the eﬀective potential and corresponds to the situation 
  where there is a single layer of water molecules between 
  the ions. The ﬁrst minimum of the potential, which cor- 
  responds to the contact ion pair (CIP) is thus completely 
  ignored by the MSA2 calculation. If the MSA diameters 
  are directly ﬁtted to reproduce the MC osmotic pres- 
  sure, much smaller values are obtained. These MSA-ﬁt 
  hydrated diameters, which are compared to the MSA2 
  diameters in the bottom part of Fig. 2, are averages of 
  the CIP and the solvent-separated ion pair. 