Density-functional calculation of the Hugoniot of shocked liquid nitrogen

We have performed molecular-dynamics (MD) simulations to obtain the internal energy and pressure of shock-compressed fluid nitrogen for over 60 different (density, temperature) points. Calculations were performed using the generalized gradient approximation in density-functional theory (DFT). The single-shock Hugoniot derived from this equation of state agrees well with gas-gun experiments for pressure vs density. As density and pressure increase along the Hugoniot, the system appears to undergo a continuous transition from a molecular to a partially dissociated fluid containing a mixture of atoms and molecules. A small fraction of clusters larger than dimers were found; however, these larger clusters were of a highly transient nature, with lifetimes of a few femtoseconds. This behavior closely resembles many features in the DFT-MD results for deuterium in the same general regime.