Molecular-dynamics simulation of the static pair-pair correlation function for classical fluids

We have used a molecular-dynamics simulation to calculate the first seven nonzero Legendre coefficients, Q(l)(r,r), of the static pair-pair correlation function Q(r,r ) . The interaction potential was taken to be Lennard-Jones. The simulations were done at two different values of density and temperature, one coinciding with that of liquid argon near its triple point and the other with liquid argon at 181.0 K. Q(r,r) can be expressed in terms of two, three-, and four-body distribution functions. We use the Kirkwood superposition approximation to estimate the three-body contribution to Q(r,r). We argue that at low densities the three-body correlations are substantially more important than the four-body correlations. The three-body part of the l=4 and 6 coefficients emphasizes configurations composed of nearest-neighbor triplets with internal angles of approximately 90°and 60°, respectively. We find that the l=6 coefficient shows a dramatic increase in structure at elevated density. Furthermore, this coefficient exhibits nearly periodic oscillations in its off-diagonal structure. Directions for future work are given.