Constant-temperature–constant-pressure molecular-dynamics calculations for molecular solids: Application to solid nitrogen at high pressure

A procedure is formulated for carrying out molecular-dynamics calculations on a molecular crystal in the (N,P,T) ensemble. As an illustration of the method the dynamical equations are used to investigate the effect on the structure of quenching solid nitrogen at P=70 kbar. An intermolecular potential model which explicitly incorporates electrostatic interactions yields a low-temperature structure which is tetragonal P4₃{2}₁$2 (Z=32). On the other hand, an atom-atom potential model without electrostatic interactions yields a low-temperature structure which is trigonal R3c (Z=64). These structures differ from the R3{}c (Z=8) structure predicted on the basis of static energy-minimization calculations [R. LeSar, J. Chem. Phys. 81, 5104 (1984); V. Chandrasekharan et al., Phys. Rev. B 28, 1095 (1983)]. Taken together, these results suggest that there are many competing structures of comparable energy and that the low-temperature, high-pressure structure of solid nitrogen is very sensitive to the choice of intermolecular potential.