  The dynamical motion of mechanical systems possesses underlying geometric
structures, and preserving these structures in numerical integration improves
the qualitative accuracy and reduces the long-time error of the simulation. For
a single mechanical system, structure preservation can be achieved by adopting
the variational integrator construction. This construction has been generalized
to more complex systems involving forces or constraints as well as to the
setting of Dirac mechanics. Variational integrators have recently been applied
to interconnected systems in Parks and Leok (2017), which are an important
class of practically useful mechanical systems whose description in terms of
Dirac structures and Dirac mechanical systems was elucidated in Jacobs and
Yoshimura (2014). Since these interconnected systems are modeled as a
collection of subsystems with forces of interconnection, we revisit some of the
properties of forced variational integrators. In particular, we derive a class
of Dirac variational integrators with forces that exhibit preservation
properties that are critical when applying variational integrators to the
discretization of interconnected Dirac systems. We close with a discussion of
ongoing and future research based on these findings.
