Atomic pair dynamics in a Lennard-Jones fluid: Comparison of theory with computer simulation

The dynamics of pairs of atoms in a dense Lennard-Jones fluid is studied by evaluating mean square displacements and related properties by computer simulation. These results are compared with theory for an anharmonic brownian oscillator and with solutions of the stochastic Langevin equation obtained using three different approximations for the pair friction coefficient tensor. It is concluded that the anharmonic oscillator model works well for short times when the initial separations are those for the first coordination shell. The stochastic Langevin equation, which involves the use of the exact average force, best reproduces the simulations when one uses a distance dependent friction tensor that contains the Oseen correction for hydrodynamic effects but also approaches zero at distances of the order of the Lennard Jones σ.