Molecular simulation data are reported that indicate that there is a simple empirical relationship between two-body and three-body interaction energies. The significance of this relationship is that three-body interactions can be estimated accurately from two-body interactions without incurring the computational penalty of three-body calculations. The relationship is tested by performing Gibbs ensemble simulations for the vapor-liquid equilibria of argon. The results are in good agreement with calculations that explicitly evaluate all three-body interactions. (C) 2000 American Institute of Physics. [S0021-9606(00)50413-9].