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A new approach to identify chemical species from molecular dynamics (MD) simulations of reacting materials under extreme temperatures and pressures is presented. The approach is based on bond-distance and vibrational criteria, derived from the examination of atomic behavior during a density functional theory MD simulation of an overdriven shock of the explosive pentaerythritol tetranitrate. For comparison, the trajectory was analyzed using popular bonding criteria commonly used in analysis of reactive MD simulations, including distance, distance-time, and bond-order criteria. Cluster analyses using the new time-dependent bond definition approach presented here and a bond-order approach revealed that species and their corresponding lifetimes were strongly dependent on the chosen approach, indicating significant implications for the development of chemical mechanisms and chemical kinetics models using the results of reactive MD simulations.

For many years there has been significant interest in polymeric phases of nitrogen at low pressure for potential application as an energetic material. This was the result of years of theoretical work indicating potentially meta-stable polymeric nitrogen. Experimental evidence of both an amorphous phase and a cubic-gauche phase has added greatly to this interest [A. F. Goncharov, E. A. Gregoryanz, H. K. Mao, Z. Liu, and R. J. Hemley, Phys. Rev. Lett. 85, 1262 (2000); M. I. Eremets, R. J. Hemley, H. K. Mao, and E. Gregoryanz, Nature (London) 411, 170 (2001)]. While most of the theoretical work has been done on the many crystal phases of nitrogen, little work has been done on simulating amorphous polymeric nitrogen. The original goal of this work was to simulate amorphous polymeric nitrogen at low pressure; however, we unexpectedly found a new form of polymeric nitrogen. Starting from first principles dynamic shock simulation of cubic-gauche nitrogen [W. D. Mattson and R. Balu, Phys. Rev. B 83, 174105 (2011)] we demonstrate a new low pressure porous form that exhibits stability at low temperatures. We describe the detailed procedure of obtaining this structure as well as some of its physical characteristics. Finally, we explore composite structures of this new form of polymeric nitrogen and their possible relationship to an amorphous form.

A rich variety of competing phases is predicted for nitrogen at accessible pressures, including a new metallic chainlike phase very close in energy to the previously predicted cubic gauche phase, and other phases at slightly higher energies, e.g., one with N2 and N6 units. Large energy barriers between structures can account for recent observations of metastability, and we identify a low barrier transition path from the known epsilon phase to the chainlike metallic phase. In analogy to MgB2, the metal is anisotropic with multiple Fermi surfaces formed from pi and sigma states.