  We study the phase behavior of polar Active Brownian Particles moving in
two-spatial dimensions and interacting through volume exclusion and velocity
alignment. We combine particle-based simulations of the microscopic model with
a simple mean-field kinetic model to understand the impact of velocity
alignment on the motility-induced phase separation of self-propelled disks. We
show that, as the alignment strength is increased, approaching the onset of
collective motion from below, orientational correlations grow, rendering the
diffusive reorientation dynamics slower. As a consequence, the tendency of
particles to aggregate into isotropic clusters is enhanced, favoring the
complete de-mixing of the system into a low and high-density phase.
