Correlations in the Motion of Atoms in Liquid Argon

A system of 864 particles interacting with a Lennard-Jones potential and obeying classical equations of motion has been studied on a digital computer (CDC 3600) to simulate molecular dynamics in liquid argon at 94.4°K and a density of 1.374 g cm^-3. The pair-correlation function and the constant of self-diffusion are found to agree well with experiment; the latter is 15% lower than the experimental value. The spectrum of the velocity autocorrelation function shows a broad maximum in the frequency region ω=0.25(k_BTℏ). The shape of the Van Hove function G_s(r, t) attains a maximum departure from a Gaussian at about t=3.0×10^-12 sec and becomes a Gaussian again at about 10^-11 sec. The Van Hove function G_d(r, t) has been compared with the convolution approximation of Vineyard, showing that this approximation gives a too rapid decay of G_d(r, t) with time. A delayed-convolution approximation has been suggested which gives a better fit with G_d(r, t) this delayed convolution makes G_d(r, t) decay as t⁴ at short times and as t at long times.