  We present a comprehensive computational study of the collective behavior
emerging from the competition between self-propulsion, excluded volume
interactions and velocity-alignment in a two-dimensionnal model of active
particles. We consider an extension of the Active Brownian Particles model
where the self-propulsion direction of the particles aligns with the one of
their neighbors. We analyze the onset of collective motion (flocking) in a
low-density regime (10% surface area) and show that it is mainly controlled by
the strength of velocity-alignment interactions: the competition between
self-propulsion and crowding effects plays a minor role in the emergence of
flocking. However, above the flocking threshold, the system presents a richer
pattern formation scenario than analogous models without alignment interactions
(Active Brownian Particles) or excluded volume effects (Vicsek-like models).
Depending on the parameter regime, the structure of the system is characterized
by either a broad distribution of finite-sized polar clusters or the presence
of an amorphous, highly fluctuating, large-scale traveling structure which can
take a lane-like or band-like form (and usually a hybrid structure which is
halfway in between both). We establish a phase diagram that summarizes
collective behavior of polar Active Brownian Particles and propose a generic
mechanism to describe the complexity of the large-scale structures observed in
systems of repulsive self-propelled particles.
