  The phase space trajectories of many body systems charateristic of simple
fluids are highly unstable. We quantify this instability by a set of Lyapunov
exponents, which are the rates of exponential divergence, or convergence, of
initial (infinitesimal) perturbations along carefully selected directions in
phase space. It is demonstrated that the perturbation associated with the
maximum Lyapunov exponent is localized in space. This localization persists in
the large-particle limit, regardless of the interaction potential. The
perturbations belonging to the smallest positive exponents, however, are
sensitive to the potential. For hard particles they form well-defined
long-wavelength modes. The modes could not be observed for systems interacting
with a soft potential due to surprisingly large fluctuations of the local
time-dependent exponents.
