  We have generalized the Nanbu collision algorithm to accommodate arbitrary
collision rates, enabling accurate kinetic modeling of short range particle
interactions in non-Spitzer systems. With this extension, we explore the effect
of different collision models on the simulation of how ultra-intense lasers
first begin to heat a target. The effect of collisions on plasma evolution is
crucial for treating particle slowing, energy transport, and thermalization.
The widely used Nanbu collision algorithm provides a fast and computationally
efficient method to include the effects of collisions between charged particles
in kinetic simulations without requiring that the particles already be in local
thermal equilibrium. However, it is "hardwired" to use Spitzer collision rates
appropriate for hot, relatively dilute plasmas. This restriction prevents the
Nanbu collision algorithm from accurately describing the initial heating of a
cold target, a key problem for the study of laser damage or the generation of
the warm dense matter state. We describe our approach for modifying the Nanbu
collision algorithm and demonstrate the improved accuracy for copper targets.
