  In amorphous solids subject to shear or thermal excitation, so-called
structural indicators have been developed that predict locations of future
plasticity or particle rearrangements. An open question is whether similar
tools can be used in dense active materials, but a challenge is that under most
circumstances, active systems do not possess well-defined solid reference
configurations. We develop a computational model for a dense active crowd
attracted to a point of interest, which does permit a mechanically stable
reference state in the limit of infinitely persistent motion. Previous work on
a similar system suggested that the collective motion of crowds could be
predicted by inverting a matrix of time-averaged two-particle correlation
functions. Seeking a first-principles understanding of this result, we
demonstrate that this active matter system maps directly onto a granular
packing in the presence of an external potential, and extend an existing
structural indicator based on linear response to predict plasticity in the
presence of noisy dynamics. We find that the strong pressure gradient
necessitated by the directed activity, as well as a self-generated free
boundary, strongly impact the linear response of the system. In low-pressure
regions the linear-response-based indicator is predictive, but it does not work
well in the high-pressure interior of our active packings. Our findings
motivate and inform future work that could better formulate structure-dynamics
predictions in systems with strong pressure gradients.
