The well-known Vicsek model describes the flock dynamics of self-propelled
agents. Surprisingly, a direct measure of the chaotic behavior of such systems
is missing. Here, we discuss the kinetic phase transition present in Vicsek
systems in light of the largest Lyapunov exponent, which is numerically
computed by following the dynamical evolution in tangent space. As
discontinuities in the neighbors weighting factor hinder the computations, we
propose a continuous form of the model. Our results about chaotic regime
reinforce the idea that the Lyapunov exponent is also a phase transition
indicator.Comment: 7 pages, 16 equations, 6 figure