  There are rich emergent phase behaviors in non-equilibrium active systems.
Flocking and clustering are two representative dynamic phases. The relationship
between these two phases is still unclear. In the paper, we numerically
investigate the evolution of flocking and clustering in a system consisting of
self-propelled particles with active reorientation. We consider the interplay
between flocking and clustering phases under different initial states, and
observe an unstable domain in order parameters phase diagrams due to initial
states even in the absence of an explicit attraction. This point is different
from the previous finding that active angular fluctuations lead to an earlier
breakdown of collective motion and the emergence of a new bi-stable regime in
the aligned active particles [R.Grossmann et al, New J. Phys.073033,14 (2012)].
In particular, we find that the existence of bi-stable states is due to the
diversity of dynamic paths arising from different initial states. By increasing
(decreasing) the initial degree of ordering, the bi-stable state can be shifted
to a more ordered flocking (disordered clustering) state. These results
enlighten us pave the way to manipulate emergent behaviors and collective
motions of active system.
