The objective of this program was to develop an accurate and fast simulation methodology for supercritical injection and mixing using molecular dynamics (MD). The use of MD allows the supercritical mixing to occur naturally' without the need for tracking phase boundaries and intrinsically includes all physical processes, material properties and equations of state in both subcritical and supercritical environments. MD also allows the tracking of injected liquid molecules into the chamber environment. Simulations of subcritical liquid nitrogen injection and atomization resulted in droplet sizes that matched Rayleigh theory. Subcritical breakup length as a function of pressure displayed behavior that matched experiments. Axial and radial density profiles for supercritical jet injection displayed the smooth transition of the density profiles from the high jet centerline to ambient chamber conditions and jet divergence angles that agreed in magnitude with steady state empirical models.