text: To overcome this diﬃculty, we have explicitly intro- 
  duced the CIP in our model (species 3). Straightforward 
  calculations, based on a characteristic-function formal- 
  ism, allow us to deﬁne an equivalent model in which 
  the free ions and the CIP are explicitly taken into ac- 
  count [19, 20]. We apply this formalism by deﬁning a 
  pair as an anion and a cation at a distance less than 
  4 ˚A, which corresponds to the position of the eﬀective 
  potential maximum. The interaction between free, like 
  charges in this new system remains unchanged, and the 
  cation-anion interactions are easily approximated by ex- 