An empirical nonreactive force field has been developed for molecular dynamics (MD)/Monte Carlo simulation of the formation, diffusion, and agglomeration of point defects in the crystal lattice of the alpha modification of hexahydro-1,3,5-trinitro-1,3,5 triazine (RDX) using flexible molecules. Bond stretching and angle bending are represented by Morse and harmonic functions, and torsion by a truncated cosine series. Nonbonded interactions, both inter- and intramolecular, are described by Buckingham potentials separately parametrized. Intermolecular electrostatic interactions are treated via a Coulomb term coupled with a smooth 15.0 A cutoff radius. Parameters were taken in part from earlier published works and were determined partly by fitting to known molecular and crystal properties of RDX. In MD simulations at constant pressure and temperature, the model was able to stabilize and maintain the correct crystal structure, symmetry, and molecular conformation of alpha-RDX. Vibrational frequencies, lattice binding energy and dimensions, coefficients of thermal expansion, and several unusually short intermolecular distances are all reproduced in satisfactory agreement with experimental data.

To help understand and predict nanotube interactions, the electrostatic potentials on both the outer and the inner surfaces of 10 single-walled model systems have been computed at the Hartree−Fock STO-5G//STO-3G level. All structures were optimized computationally. Both carbon and boron/nitrogen tubes were studied, including the open and closed (5,5) and the open (6,1), (7,1), and (8,1), plus fullerene for comparison. Hydrogen atoms were introduced at the ends of the open tubes, to satisfy the unfulfilled valencies. The surface potentials were characterized in terms of both site-specific and global properties: positive and negative extrema and average values, average deviation, positive and negative variances, and electrostatic balance. The all-carbon systems, the closed (5,5) and fullerene, are very weakly positive on most of the outer and all of the inner surfaces, the latter potentials being somewhat stronger. In contrast, the open carbon tubes with charge-donating hydrogens at the ends are slightly negative on the outer lateral surfaces and somewhat less so on the inner, except for the narrowest, the (6,1), which is positive inside. The boron/nitrogen tubes have much stronger and more variable surface potentials than do the carbon; there are characteristic patterns of positive and negative sites on the outer lateral surfaces, while the inner ones are markedly positive. A general feature of all of the systems studied, both carbon and boron/nitrogen, is that stronger potentials are associated with regions of higher curvature.

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