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.

Molecular dynamics simulations are performed to determine the melting points of aluminum nanoparticles of 55-1000 atoms with the Streitz-Mintmire [Phys. Rev. B 1994, 50, 11996] variable-charge electrostatic plus potential. The melting of the nanoparticles is characterized by studying the temperature dependence of the potential energy and Lindemann index. Nanoparticles with less than 850 atoms show bistability between the solid and liquid phases over temperature ranges below the point of complete melting. The potential energy of a nanoparticle in the bistable region alternates between values corresponding to the solid and liquid phases. This bistability is characteristic of dynamic coexistence melting. At higher temperatures, only the liquid state is stable. Nanoparticles with more than 850 atoms undergo a sharp solid-liquid-phase transition characteristic of the bulk solid phase. The variation of the melting point with the effective nanoparticle radius is also determined.

A previously developed intermolecular potential for nitramines and several other classes of nitrocompound crystals has been used to investigate the behavior of the energetic materials hexahydro-l, 3,3, l,3,5,3,5,2,4,6,8,10,, and pentaerythritol tetranitrate (PETN) under hydrostatic compression. Isothermal-isobaric molecular simulations (assuming the rigid-molecule approximation) molecular-packing calculations were used to perform the analyses. In the case of the RDX, HMX, and HNIW crystals, the results indicate that the proposed potential model is able to accurately reproduce the changes in the structural crystallographic parameters as functions of pressure for the entire range of pressures that has been investigated experimentally. In addition, the calculated bulk moduli of RDX and HMX were found to be in good agreement with the corresponding experimental results. In the case of the PETN crystal, the crystallographic parameters have been reproduced with an acceptable accuracy at pressures up to about 5 GPa. The larger deviations from the experimental results at greater pressures indicate the limitations of the rigid-molecule model when applied to floppy molecules. The similarity of the results determined in molecular-packing calculations relative to those from molecular dynamics simulations suggest that the former method can be used as an efficient tool for rapid tests of the crystal structure modification under pressure.n Acknowledgments

W91INF-05-1-0265 REPORT NUMBER REPORT ~'u~mER 48101-EG-MUR The views, opinions andlor findings contained in this report are those of the author(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other documentation. Approved for public release; federal purpose rights The response of the energetic molecular crystal cyclotrimethylene trinitramine (RDX) to the propagation of planar shock waves nonnal to (100) has been studied using large-scale molecular dynamics simulations that employ an accurate and transferable nonreactive potential. The propagation of the shock waves was simulated using nonequilibrium molecular dynamics. Shear bands were nucleated during shocks with a particle velocity of 1.0 km s-I and corresponding Rankine-Hugoniot shock pressun of9.7 GPa. These defects propagate into the compressed material at 45° to (100) in the (010) zone. The shear bands e\'oh'e slowly compared to the time scales routinely accessible to nonequilibrium molecular dynamics toward a liquidlike state as a result of viscous heating. A recently developed shock-front absorbing boundary condition [AV. Bolesta et al. , Phys. Rev. B 76, 224108 (2007)] was applied to the simulation cells at the moment of maximum compression to sustain the shock-compressed state. Molecular dynamics simulations were then employed to study the temIXlral and structural evolution of the shock-induced shear bands toward a steady-fluctuating state. Owing to the intense, viscous flow-driven heati ng within the shear bands. these defects can be considered to be homogeneously nucleated hot spots.

An experiment was conducted to determine the effects of high vs low body condition scores (BCS) produced by restricted feeding on reproductive characteristics, hormonal secretion, and leptin concentrations in mares during the autumnal transition and winter anovulatory period. Mares with BCS of 6.5 to 8.0 were maintained on pasture and/or grass hay, and starting in September, were full fed or restricted to produce BCS of 7.5 to 8.5 (high) or 3.0 to 3.5 (low) by December. All but one mare with high BCS continued to ovulate or have follicular activity during the winter, whereas mares with low BCS went reproductively quiescent. Plasma leptin concentrations varied widely before the onset of restriction, even though all mares were in good body condition. During the experiment, leptin concentrations gradually decreased (P < 0.0001) over time in both groups, but were higher (P < 0.009) in mares with high vs low BCS after 6 wk of restriction, regardless of initial concentration. No differences (P > 0.1) between groups were detected for plasma concentrations of LH, FSH, TSH, GH, glucose, or insulin in samples collected weekly; in contrast, plasma prolactin concentrations were higher (P < 0.02) in mares with high BCS, but also decreased over time (P < 0.008). Plasma IGF-I concentrations tended (P = 0.1) to be greater in mares with high vs low BCS. The prolactin response to sulpiride injection on January 7 did not differ (P > 0.1) between groups. During 12 h of frequent blood sampling on January 12, LH concentrations were higher (P < 0.0001), whereas GH concentrations (P < 0.0001) and response to secretagogue (EP51389; P < 0.03) were lower in mares with high BCS. On January 19, the LH response to GnRH was higher (P < 0.02) in mares with high BCS; the prolactin response to TRH also was higher (P < 0.01) in mares with high BCS. In conclusion, nutrient restriction resulting in low BCS in mares resulted in a profound seasonal anovulatory period that was accompanied by lower leptin, IGF-I, and prolactin concentrations. All but one mare with high BCS continued to cycle throughout the winter or had significant follicular activity on the ovaries. Although leptin concentrations on average are very low in mares with low BCS and higher in well-fed mares, there is a wide variation in concentrations among well-fed mares, indicating that some other factor(s) may determine leptin concentrations under conditions of high BCS.

Gaussian2 theory: Use of higher level correlation methods, quadratic configuration interaction geometries, and secondorder Mo/ller-Plesset zeropoint energies