  Cascade solutions of the Boltzmann equation suffer from causality violation
at large densities and/or scattering cross sections. Although the particle
subdivision technique can reduce the causality violation, it alters
event-by-event correlations and fluctuations and is also computationally
expensive. Here we evaluate and then improve the accuracy of the ZPC parton
cascade for elastic scatterings inside a box without using parton subdivision.
We first test different collision schemes for the collision times and ordering
time and find that the default collision scheme does not accurately describe
the equilibrium momentum distribution at large opacities. We then find a
specific collision scheme that can describe very accurately the equilibrium
momentum distribution as well as the time evolution towards equilibrium, even
at large opacities. We also calculate the shear viscosity and the $\eta/s$
ratio of the parton systems and confirm that the new collision scheme is more
accurate. In addition, we use a novel parton subdivision method to obtain the
"exact" evolution of the system. This subdivision method is valid for such box
calculations and is so much more efficient than the standard subdivision method
that we use a subdivision factor of $10^6$ in this study.
