We have developed an intermolecular potential that describes the structure of the α-form of the hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) crystal. The potential is composed of pairwise atom−atom (6-exp) Buckingham interactions and charge−charge interactions. The parameters of the Buckingham repulsion−dispersion terms have been determined through a combination of nonlinear least-squares fitting to observed crystal structures and lattice energies and trial-and-error adjustment. Crystal packing calculations were performed to determine the equilibrium crystallographic structure and lattice energy of the model. There are no significant differences in the geometrical structures and crystal energies resulting from minimization of the lattice energy with and without symmetry constraints. Further testing of the intermolecular potential has been done by performing symmetry-constrained isothermal−isobaric Monte Carlo simulations. The properties of the crystal (lattice dimensions, molecular orientation, and lattice energy) determined from Monte Carlo simulations at temperatures over the range 4.2−300 K indicate good agreement with experimental data. The intermolecular potential was also subjected to isothermal−isobaric molecular dynamics calculations at ambient pressure for temperatures ranging from 4.2 to 325 K. Crystal structures at 300 K are in outstanding agreement with experiment (within 2% of lattice dimensions and almost no rotational and translational disorder of the molecules in the unit cell). The space-group symmetry was maintained throughout the simulations. Thermal expansion coefficients were determined for the model and are in reasonable accord with experiment.

First-principles pseudopotential plane wave calculations based on spin-polarized density functional theory ͑DFT͒ and the generalized gradient approximation ͑GGA͒ have been used to study the adsorption of CO molecules on the Fe͑100͒ surface. Among several possible adsorption configurations considered here, the most stable corresponds to a fourfold state in which a CO molecule is tilted relative to the surface normal by 50°. In this case, the CO bond is elongated to 1.32 Å and has a low vibrational stretching frequency of 1246 cm Ϫ1 to be compared with the experimental gas phase value of 2143 cm Ϫ1 . The adsorption energy for this state is found to vary between 46.7 and 43.8 kcal/mol depending on the choice of exchange-correlation functional used in the DFT. A total of three adsorption sites have been located, and the relative adsorption energies are E(fourfold)ϾE(twofold)ϷE(onefold) at lower surface coverage, and E(fourfold)ϾE(onefold) ϾE(twofold) at higher coverage. A similar analysis performed for the C and O atoms indicates that the adsorption at the fourfold site is the most stable among various configurations, with adsorption energies of 186 and 145 kcal/mol, respectively. Additionally, we have demonstrated the possibility that a C atom embeds into the lattice in a twofold, bridgelike configuration with an adsorption energy of 154 kcal/mol. The minimum energy pathways for the surface diffusion of a CO molecule between selected pairs of local minima indicate that the barriers for these processes are generally quite small with values less than 2 kcal/mol. One exception to this is the diffusion out of the most stable fourfold site, where the barrier is predicted to be around 13 kcal/mol. Finally, the barriers for dissociation of CO bound in a fourfold site have been calculated to have values in the range of 24.5-28.2 kcal/mol, supporting the experimental observation that dissociation of CO bound to the surface seems to compete with CO desorption at 440 K.

A classical potential to simulate the dynamics of a nitromethane crystal as a function of temperature and pressure is described. The intramolecular part of the potential was taken as superposition of bond stretching, bond bending, and torsional angles terms. These terms were parametrized on the basis of the geometric and spectroscopic (vibrational frequencies and eigenvectors) data obtained using ab initio molecular orbital calculations performed at the B3LYP/6-31G* level on an isolated molecule. The intermolecular potential used is of the Buckingham 6-exp form plus charge−charge Coulombic interactions and has been previously developed by us (Sorescu, D. C.; Rice, B. M.; Thompson, D. L. J. Phys. Chem. 1997, B101, 798) to simulate crystals containing nitramine molecules and several other classes of nitro compounds. The analyses performed using constant pressure and temperature molecular dynamics simulations and molecular packing calculations indicate that the proposed potential model is able to reproduce accurately the changes of the structural crystallographic parameters as functions of temperature or pressure for the entire range of values investigated. In addition, the calculated bulk modulus of nitromethane was found in excellent agreement with the corresponding experimental results. Moreover, it was determined that the present potential predicts correctly an experimentally observed 45° change in methyl group orientation in the high-pressure regime relative to the low-temperature configuration. The analysis of the linear expansion coefficients and linear compression data indicate anisotropic behavior for the unit cell edges.