Dynamical phase transitions in supercooled liquids: interpreting measurements of dynamical activity.

We study dynamical phase transitions in a model supercooled liquid. These transitions occur in ensembles of trajectories that are biased towards low (or high) dynamical activity. We compare two different measures of activity that were introduced in recent papers and we find that they are anti-correlated with each other. To interpret this result, we show that the two measures couple to motion on different length and time scales. We find that "inactive" states with very slow structural relaxation nevertheless have increased molecular motion on very short scales. We discuss these results in terms of the potential energy landscape of the system and in terms of the liquid structure in active/inactive states.