The relative permittivity in equation ( ) is assumed to vary with cation concentration as given by equation ( ) . The concentration terms in the equation (12) are in mol dm -3 and M water is the molarity of water at room temperature taken as 55 M. The parameter w i is the total number of water molecules held by the ion i. ε 0 r is taken to be 80.1 equal to the relative permittivity of water at room temperature whereas ε min r is the dielectric constant of water under the condition of dielectric saturation and is taken equal to 6. The model in equation ( ) is thus a summation of the contributions of bulk and cation-bounded water molecules to the relative permittivity. More advanced models such as the Booth model describe the dependence of the relative permittivity of the electrolyte medium as a function of the electric field . However, the Booth model is numerically more challenging to solve simultaneously with the GMPNP due to its non-linear nature.