  Using molecular dynamics computer simulations we investigate the
out-of-equilibrium dynamics of a Lennard-Jones system after a quench from a
high temperature to one below the glass transition temperature. By studying the
radial distribution function we give evidence that during the aging the system
is very close to the critical surface of mode-coupling theory. Furthermore we
show that two-time correlation functions show a strong dependence on the
waiting time since the quench and that their shape is very different from the
one in equilibrium. By investigating the temperature and time dependence of the
frequency distribution of the normal modes we show that the energy of the
inherent structures can be used to define an effective (time dependent)
temperature of the aging system.
