By molecular dynamics for a Lennard-Jones fluid at $n^{\mathrm{*}}$=0.55 and $T^{\mathrm{*}}$=3.2 we determine both the Fourier-transformed velocity autocorrelation function z(ω) and the incoherent neutron scattering functions $S_s$(k,ω). $z( omega )— (including the ‘‘long-time-tail’’ singularity near ω=0) can be obtained from $S_s$(k,ω) by extrapolation to k=0, only when data for $S_s$(k,ω) are included with k smaller than (ω/2D${)}^{1/2}$, with D the self-diffusion coefficient.

• Received 12 October 1988

DOI:https://doi.org/10.1103/PhysRevA.39.2731