Role of Repulsive and Attractive Forces in Liquids The Optimized Random Phase Approximation

The optimized random phase approximation (OPPA) Is applied to the calculation of the thermodynamlc properties and pair correlation function of slmole liquids. General formulas are presented, together with results tor the Lennard-Jones fluid, which are compared with Monte Carlo and molecular dynamic results. Excellent agreement Is obtained for the entire single phase fluid region of the phase diagram, except for the critical region and the very low temperature vapor. The theory converges especially rapidly at high densities or at high temperatures. The Importance of separating the intc-molecular potential In the proper way to obtain most rapid convergence of perturbation theories Is discussed. It is concluded that the structure of simple liquids Is determined mostly by the rapidly varying parts of the potential. Further, the ORPA provides an accurate theory for calculating the contributions from different forces to the structure and thermodynamics of liquids.