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Experiments involving the coupling of metal and high explosives (HE) are of notable defense-related interest, and we seek to refine the uncertainty quantification associated with models of such experiments. In particular, our focus is on how uncertainty related to the metal constitutive model challenges our ability to infer high explosive model parameters when analyzing focused science experiments. We consider three focused experiments involving an HE accelerating metal: small plate tests with tantalum/LX-14 and tantalum/LX-17 pairings as well as a tantalum/LX-17 cylinder test. For all three models, we perform sensitivity analysis to ascertain the influence of metal strength on the coupled experimental response. Moreover, we calibrate each model in a Bayesian setting and study the quantification of metal strength on the inference of the HE parameters. Based on our results, we offer guidance for future metal/HE experiments. [ABSTRACT FROM AUTHOR]

We present a comprehensive simulation study on the solid-liquid phase transition of the ionic liquid 1,3-dimethylimidazolium chloride in terms of the changes in the atomic structure and their effect on the Compton profile. The structures were obtained by using ab initio molecular dynamics simulations. Chosen radial distribution functions of the liquid structure are presented and found generally to be in good agreement with previous ab initio molecular dynamics and neutron scattering studies. The main contributions to the predicted difference Compton profile are found to arise from intermolecular changes in the phase transition. This prediction can be used for interpreting future experiments. [ABSTRACT FROM AUTHOR]

First-principles molecular dynamics simulations, in which the forces are computed from electronic structure calculations, have great potential to provide unique insight into structure, dynamics, electronic properties, and chemistry of interfacial systems that is not available from empirical force fields. The majority of current first-principles simulations are driven by forces derived from density functional theory with generalized gradient approximations to the exchange-correlation energy, which do not capture dispersion interactions. We have carried out first-principles molecular dynamics simulations of air-water interfaces employing a particular generalized gradient approximation to the exchange-correlation functional (BLYP), with and without empirical dispersion corrections. We assess the utility of the dispersion corrections by comparison of a variety of structural, dynamic, and thermodynamic properties of bulk and interfacial water with experimental data, as well as other first-principles and force field-based simulations. [ABSTRACT FROM AUTHOR]

We report herein first principles simulations of water under shock loading and the chemical reactivity under these hot, compressed conditions. Using a recently developed simulation technique for shock compression, we observe that water achieves chemical equilibrium in less than 2 ps for all shock conditions studied. We make comparison to the experimental results for the Hugoniot pressure and density final states. Our simulations show that decomposition occurs through the reversible reaction H2O↔H++OH-, in agreement with experiment. Near the approximate intersection of the Hugoniot and the Neptune isentrope, we observe high concentrations of charged species that contribute electronic states near the band gap. [ABSTRACT FROM AUTHOR]

We report herein ab initio molecular dynamics simulations of graphite under shock compression in conjunction with the multiscale shock technique. Our simulations reveal that a novel short-lived layered diamond intermediate is formed within a few hundred of femtoseconds upon shock loading at a shock velocity of 12 km/s (longitudinal stress>130 GPa), followed by formation of cubic diamond. The layered diamond state differs from the experimentally observed hexagonal diamond intermediate found at lower pressures and previous hydrostatic calculations in that a rapid buckling of the graphitic planes produces a mixture of hexagonal and cubic diamond (layered diamond). Direct calculation of the x-ray absorption spectra in our simulations reveals that the electronic structure of the final state closely resembles that of compressed cubic diamond. [ABSTRACT FROM AUTHOR]

A research addressing a comprehensive particle-based simulation study of the structural, dynamic, and electronic properties of the liquid-vapor interface of water utilizing both ab initio and empirical models is illustrated. Numerous properties such as interfacial width, hydrogen bond populations, dipole moments, and correlation times will be characterized with identical schemes to draw useful conclusions on the strengths and weakness of the proposed models for interfacial water.

First principles Monte Carlo simulations in the Gibbs and isobaric–isothermal ensembles were performed to map the vapor–liquid coexistence curves of methanol and methane described by Kohn–Sham density functional theory using the Becke–Lee–Yang–Parr (BLYP) exchange and correlation functionals with the Grimme correction term for dispersive (D2) interactions. The simulations indicate that the BLYP-D2 description with the TZV2P basis set underpredicts the saturated vapor densities and overpredicts the saturated liquid densities and critical and boiling temperatures for both compounds. Although the deviations are quite large, these results present a significant improvement over the BLYP functional without the correction term, which misses the experimental results by a larger extent in the opposite direction. Simulations at one temperature indicate that use of the larger QZV3P basis set may lead to improved saturated vapor densities, but not to significant changes in the liquid density. [ABSTRACT FROM AUTHOR]

Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlikely mechanism for the origins of life because of unfavourable chemical conditions on the planet and the high heat from impact. In contrast, we find that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine. Our ab initio molecular dynamics simulations show that shock waves drive the synthesis of transient C-N bonded oligomers at extreme pressures and temperatures. On post impact quenching to lower pressures, the oligomers break apart to form a metastable glycine-containing complex. We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet. [ABSTRACT FROM AUTHOR]

We present herein a comprehensive density functional theory study toward assessing the accuracy of two popular gradient-corrected exchange correlation functionals on the structure and density of liquid water at near ambient conditions in the isobaric-isothermal ensemble. Our results indicate that both PBE and BLYP functionals under predict the density and over structure the liquid. Adding the dispersion correction due to Grimme (1, 2) improves the predicted densities for both BLYP and PBE in a significant manner. Moreover, the addition of the dispersion correction for BLYP yields an oxygen-oxygen radial distribution function in excellent agreement with experiment. Thus, we conclude that one can obtain a very satisfactory model for water using BLYP and a correction for dispersion.

We have studied the OHCl(-) complex in a six-water cluster and in bulk liquid water by means of Born−Oppenheimer molecular dynamics based on generalized gradient-corrected BLYP density functional theory. Self-interaction-corrected results, which predict a hydrogen-bonded OH···Cl(-) complex, are compared to the uncorrected results, which predict a hemibonded (HO-Cl)(-). A second-order Møller−Plesset potential energy landscape of the gas-phase complex in its ground-state was computed to determine which of the two configurations represents the true nature of the complex. Because no evidence of a local minimum was found in the vicinity of the geometry corresponding to (HO-Cl)(-), we conclude that the self-interaction-corrected results are more accurate and, therefore, that the complex is held together by a hydrogen-bond-like interaction in both an asymmetric solvation environment, as represented by the cluster, and a symmetric solvation environment, as represented by the bulk system. We postulate that the mechanism that governs the atmospheric oxidation of Cl(-)(aq) to Cl(2)(g) on the surface of marine aerosols is initiated by the formation of a H-bonded OH···Cl(-) complex. Furthermore, because no evidence of charge transfer from Cl(-) to OH was found, in either the liquid or the cluster environment, we propose that the second step of the oxidation of Cl(-) is the reaction of the complex with a second Cl(-), resulting in the formation of the species Cl(2)(-) and OH(-). Cl(2)(g) could then be formed via an electron-transfer reaction with an impinging OH molecule.

Background/aim: Cardiovascular disease is the major cause of mortality in dialysis patients, accounting for about 40% of deaths in most large registries. Oxidative stress has been strongly implicated in the pathogenesis of these events. As end-stage renal disease is a state of elevated free radical activity, the aim of the present study was to investigate the negative impact of smoking in 57 male hemodialysis patients., Methods: The patients, who were 20-85 years of age (mean age 51.0 +/- 14 years), had been on hemodialysis for at least 6 months before participating in this study. Fasting blood sampling for serum lipid, albumin, urate and lipophilic antioxidants such as tocopherols, carotenes, ascorbate and lipid peroxides was performed., Results: The plasma malondialdehyde (MDA) concentration was significantly higher in hemodialysis patients who smoked compared to hemodialysis patients who were nonsmokers (1.92 +/- 0.52 vs. 1.59 +/- 0.42 nmol/ml, p = 0.006). No association was found between levels of MDA in smokers and parameters such as body mass index, serum cholesterol, serum triglycerides and smoking index. There were no significant differences in the plasma levels of uric acid, alpha-tocopherol, gamma-tocopherol, delta-tocopherol, alpha-carotene, beta-carotene and retinol between the two groups. A significantly lower level of plasma ascorbate was observed in hemodialysis patients who smoked compared to the nonsmoking hemodialysis patients or healthy controls (4.59 +/- 4.0 vs. 9.57 +/- 4.0 and 10.16 +/- 4.6 microg/ml, p < 0.05). Moreover, in smokers, the plasma levels of ascorbate were negatively correlated with the levels of plasma MDA (r = -0.43, p < 0.001) of each patient. Partial correlation analysis of the plasma levels of the measured antioxidants and the smoking index revealed a negative correlation between the plasma levels of lipid-normalized lycopene and the smoking index (r = -0.53, p < 0.05)., Conclusion: Our data suggest that cigarette smoking further increases plasma-circulating products of lipid peroxidation, which are already increased in nonsmoking hemodialysis patients as compared to matched healthy controls. The lower plasma levels of ascorbate in hemodialysis patients who smoke suggest that these patients may be more susceptible to oxidative tissue damage caused by smoking., (Copyright 2001 S. Karger AG, Basel)