Simulations of argon diffusion and of picosecond recombination of nitric oxide to myoglobin are presented. Three computational methods are compared, usual classical trajectories, mean field trajectories, and mean field trajectories with a binary collision correction. The ligand mobility and the spatial distribution of the ligand in the protein interior are examined. Time-dependent properties calculated with the binary collision model are in good agreement with exact simulations. This is to be contrasted with the original mean field protocol that is not working well for dynamics