  The connectivity of the potential energy landscape in supercooled atomic
liquids is investigated through the calculation of the instantaneous normal
modes spectrum and a detailed analysis of the unstable directions in
configuration space. We confirm the hypothesis that the mode-coupling critical
temperature is the $T$ at which the dynamics crosses over from free to
activated exploration of configuration space. We also report the observed
changes in the local connectivity of configuration space sampled during aging,
following a temperature jump from a liquid to a glassy state.
