Implications of many-body forces for equations of state at high pressure

In this paper we review our recent work concerned with assessing the significance of many-body forces for short-range interactions of closed shell atoms and molecules.Ab initio molecular orbital calculations (of the supermolecule type) have been carried out to determine three-body potentials for the following species: He, Ne, H2, N2, CO and CO2. For He, Ne, H2 and N2 we have also carried out calculations of the four-body potential for a limited number of orientations. These studies show that, for all these species, there are significant deviations from pair-wise additivity at short separations. The effect of these many-body interactions on the equation of state for dense fluids (such as detonation products) is being investigated by Monte Carlo simulation, and recent results for high-pressure helium are described.