The direct numerical simulations of Lennard-Jones liquids show that, based on the magnitudes of the longitudinal Ž . convection and local acceleration spectrums, the k,w space between ; 1 -kD -; 40 can be divided into two regions, where k is the wave number, w is the frequency and D is the particle diameter. In the first region, the convection dominates, and in the second region the local acceleration dominates. Along the surface dividing these two regions, there is a sudden change in the phase of longitudinal force spectrum relative to the phase spectrums of the longitudinal local acceleration, longitudinal convection acceleration and number density. It is shown that this sudden change in the phase implies that the correlated part of longitudinal force spectrum is zero along the above surface. The computed averaged magnitude force spectrum along this surface is locally minimum -but nonzero -because it contains an uncorrelated part that cannot be removed by averaging. Thus, along the above surface the convection and local acceleration densities balance each other, and the particle dynamics is similar to that of a free particle subjected to an uncorrelated 'random' force.