Coarse-grained potentials from Widom's particle insertion method

Widom's particle insertion method is used to calculate effective pair potentials for binary A/B mixtures, where B molecules represent a solvent whose degrees of freedom are integrated out. Comparison of Widom's method with an alternative route via the potential of mean force shows good agreement; however, Widom's particle insertion method experiences sampling difficulties at high densities. An isotherm for a binary mixture of argon ('solvent', B) and krypton ('solute', A) is calculated. While the structure at the pair level is well represented by the effective system using only effective pair potentials and ignoring all higher multi-body contributions, increasing deviations in the pressure are observed as the density of the effective system is increased. These latter deviations are attributed to the neglect of three- and higher multi-body interactions.