  Simulating photon dynamics in strong light-matter coupling situations via
classical trajectories is proving to be powerful and practical. Here we analyze
the performance of the approach through the lens of the exact factorization
approach. Since the exact factorization enables a rigorous definition of the
potentials driving the photonic motion it allows us to identify that the cause
of the underestimation of photon number and intensities observed in earlier
work is primarily due to an inadequate accounting of light-matter correlation
in the classical Ehrenfest force rather than errors from treating the photons
quasiclassically per se. The latter becomes problematic when the number of
photons per mode begins to exceed a half.
