In many situations a more detailed representation of the solvent molecules seems necessary and the simplest possible model for the solvent, which retains particulate structure, is known as the solvent primitive model (SPM). In this simple model, which is essentially an extension of the PM to multicomponent form, the solvent particles are treated as neutral hard-spheres of finite size. Although the SPM is clearly an oversimplification in its description of the solvent molecules, an important improvement over PM electrolytes has been observed particularly for systems of high electrolyte concentration where the exclusion packing effects and the short-ranged repulsive interactions are increasingly significant.