Computer simulation and theory for free energies in dilute near‐critical solutions

Computer simulation results are presented for the residual chemical potential μ 1r, local solvent density enhancement Δρ_loc, and average solute potential energy u₁ of a solute molecule immersed in afluid very close to its critical point. The simulation algorithm involves the Monte Carlo method in conjunction with free energy perturbation ideas. A number of ideas related to the feasibility of computer simulations in near‐critical solvents are investigated. Due to long‐range correlations near the critical point, it is not clear that simulations in this regime can be expected to yield accurate results using ensemble sizes of the magnitude typically used in simulations. Examination of system size and number of sampling steps on the simulations shows that the accuracy of the results depend to a large extent on the nature of the thermodynamic property being investigated. The solute chemical potential is suited particularly to simulation since it depends primarily on short‐range structure in the system. A comparison of the simulation results with integral equation theory calculations shows both approaches agree well with each other.