  We show that self-driven particles coupled to a periodic obstacle array
exhibit novel active matter commensuration effects that are absent in the
Brownian limit. As the obstacle size is varied for sufficiently large activity,
a series of commensuration effects appear in which the motility induced phase
separation produces commensurate crystalline states, while for other obstacle
sizes we find frustrated or amorphous states. The commensuration effects are
associated with peaks in the amount of six-fold ordering and the maximum
cluster size. When a drift force is added to the system, the mobility contains
peaks and dips similar to those found in transport studies for commensuration
effects in superconducting vortices and colloidal particles.
