  A fully variational, unstructured, electromagnetic particle-in-cell
integrator is developed for integration of the Vlasov-Maxwell equations. Using
the formalism of Discrete Exterior Calculus, the field solver, interpolation
scheme and particle advance algorithm are derived through minimization of a
single discrete field theory action. As a consequence of ensuring that the
action is invariant under discrete electromagnetic gauge transformations, the
integrator exactly conserves Gauss's law.
