A link between two-body and three-body interaction energies in fluids

Advances in molecular simulation algorithms coupled with rapid growth in the calculation speed of modern high performance computers, provide unprecedented opportunities to investigate natural phenomena from a molecular perspective. Molecular simulation has been applied to the design of beneficial pharmaceutical products, the optimisation of important industrial processes, and to the resolution of fundamental scientific questions. Despite the use of high performance computing, molecular simulation is confined largely to the calculation of two-body interactions using 'effective' intermolecular potentials because the inclusion of three- or more-body interactions remain computationally prohibitive. Generally, interactions between pairs of molecules make the overwhelming contribution to the overall intermolecular interaction. However, it is also documented that three-body interactions can make a significant contribution to intermolecular interactions in liquids. The use of 'effective' intermolecular potentials is a source of considerable inaccuracy and uncertainty in molecular simulations. For example, recent calculations have shown that three-body interactions contribute significantly to the phase behaviour of fluids, whereas this effect had been hidden previously by the use of effective intermolecular potentials. The agreement between experiment and theory for the phase envelope was improved considerably by explicitly accounting for three-body interactions. In this work, we discuss recently reported molecular simulation data which indicate there is a simple empirical relationship between two body and three-body interaction energies for noble gas atoms. 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 has the potential of improving both the accuracy and predictive value of molecular simulation