Comment on “On the Accuracy of Force Fields for Predicting the Physical Properties of Dimethylnitramine”

Polarizable and nonpolarizable force fields for alkyl nitrates

Quantum-chemistry-based many-body polarizable and two-body nonpolarizable atomic force fields were developed for alkyl nitrate liquids and pentaerythritol tetranitrate (PETN) crystal. Bonding, bending, and torsional parameters, partial charges, and atomic polarizabilities for the polarizable force field were determined from gas-phase quantum chemistry calculations for alkyl nitrate oligomers and PETN performed at the MP2/aug-cc-pvDz level of theory. Partial charges for the nonpolarizable force field were determined by fitting the dipole moments and electrostatic potential to values for PETN molecules in the crystal phase obtained from molecular dynamics simulations using the polarizable force field. Molecular dynamics simulations of alkyl nitrate liquids and two polymorphs of PETN crystal demonstrate the ability of the quantum-chemistry-based force fields to accurately predict thermophysical and mechanical properties of these materials.

Molecular dynamics simulations of the melting mechanisms of perfect and imperfect crystals of dimethylnitramine

The melting mechanisms of perfect and imperfect crystalline dimethylnitramine have been studied using molecular dynamics simulations. The imperfect crystal was created by removing approximately 10% of the molecules from the center of the simulation cell. The density, diffusion coefficient, translational and orientational order parameters, and void size were calculated as functions of temperature and simulation time. Upon melting, the volume of the imperfect crystal slowly decreases with time due to the shrinkage of the void then suddenly decreases to a minimum value due to collapse of the structure around the void with concomitant diffusion of molecules into the void. The simulation cell volume then increases as the liquid nucleus formed at the void expands. The melting of perfect crystals must occur by a different mechanism. As the temperature of the perfect crystal reaches the maximum superheating temperature, there is an increase in the thermal motions of the molecules that result in the formation of liquid centers (characterized by translational order parameter consistent with the liquid phase) at random locations. The liquid centers rapidly grow, resulting in a complete transition to the liquid phase. The increases in orientational and translational freedom occur simultaneously in the imperfect crystal, and in the perfect crystal, orientational freedom significantly precedes translational freedom.