  In this work, the short-time dynamics of simple liquid is explored both
analytically and numerically with the focus on the interplay between the
density fluctuations in a volume surrounding a chosen particle and its random
walk motion. The particles interact via the Lennard-Jones potential with
parameters corresponding to liquid argon. For large times, analytical
calculations based on the fluctuation theory provides an explicit expression
reproducing isothermal change of the self-diffusion coefficient in liquid argon
corresponding to the experimental data. These results lead to the conclusion
that such behavior is based on the reduced mobility of particles reflected in
their density fluctuations that can be equivalently achieved in the cases of
either low temperatures and pressures (supercooling) or moderate temperatures
and high pressures (overdamping).
