Laurence E. Fried

Simulation of the intermolecular vibrational spectra of liquid water and water clusters

We report simulated Raman and infrared spectra of liquid water and water clusters in the frequency range 0–1000 cm−1. The librational peak in the Raman spectrum of the liquid, which has a strong dependence on the anisotropy of the assumed gas‐phase polarizability tensor, allows us to choose between various models for that tensor. Most of the spectroscopically probed dynamics of the liquid are present in the small clusters, with N as low as 5. The librational peaks in the pentamer spectra are shown to redshift with increasing temperature.We report simulated Raman and infrared spectra of liquid water and water clusters in the frequency range 0–1000 cm−1. The librational peak in the Raman spectrum of the liquid, which has a strong dependence on the anisotropy of the assumed gas‐phase polarizability tensor, allows us to choose between various models for that tensor. Most of the spectroscopically probed dynamics of the liquid are present in the small clusters, with N as low as 5. The librational peaks in the pentamer spectra are shown to redshift with increasing temperature.

Semiclassical quantization using classical perturbation theory: Algebraic quantization of multidimensional systems

The method of algebraic quantization, a semiclassical analog of Van Vleck perturbation theory, is applied to multidimensional resonant, nonresonant, and nearly resonant systems. perturb, a special purpose program written in C, is utilized to implement classical perturbation theory efficiently to high order. States corresponding to both regular and chaotic classical regimes are quantized, and accurate eigenvalues obtained in both cases. Various quantization rules are compared, and a novel symmetry preserving rule is given which leads to good agreement with quantum mechanics. The method is able to reproduce purely quantum mechanical splittings to very good accuracy. Algebraic quantization combined with Pade resummation is used to determine energy eigenvalues for a resonant system with five degrees of freedom.

Nitrogen-Rich Heterocycles as Reactivity Retardants in Shocked Insensitive Explosives

We report the first quantum-based multiscale simulations to study the reactivity of shocked perfect crystals of the insensitive energetic material triaminotrinitrobenzene (TATB). Tracking chemical transformations of TATB experiencing overdriven shock speeds of 9 km/s for up to 0.43 ns and 10 km/s for up to 0.2 ns reveal high concentrations of nitrogen-rich heterocyclic clusters. Further reactivity of TATB toward the final decomposition products of fluid N(2) and solid carbon is inhibited due to the formation of these heterocycles. Our results thus suggest a new mechanism for carbon-rich explosive materials that precedes the slow diffusion-limited process of forming the bulk solid from carbon clusters and provide fundamental insight at the atomistic level into the long reaction zone of shocked TATB.

Solvation structure and the time‐resolved Stokes shift in non‐Debye solvents

We develop a microscopic theory of the time‐resolved Stokes shift of a chromophore in a polar solvent which incorporates both non‐Debye dielectric relaxation and solvation shell structure. The present theory depends on the direct correlation function of the pure solvent, the measured frequency‐dependent dielectric constant, and a microscopically derived translational diffusion parameter. We compare the predictions of the theory given here to a variety of experimental results on solvation in protic and aprotic solvents. Good agreement with experiment is found. Our theory compares favorably with the dynamical mean spherical approximation (MSA) theory of time‐dependent solvation.

Ab initio based force field and molecular dynamics simulations of crystalline TATB

An all-atom force field for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) is presented. The classical intermolecular interaction potential for TATB is based on single-point energies determined from high-level ab initio calculations of TATB dimers. The newly developed potential function is used to examine bulk crystalline TATB via molecular dynamics simulations. The isobaric thermal expansion and isothermal compression under hydrostatic pressures obtained from the molecular dynamics simulations are in good agreement with experiment. The calculated volume-temperature expansion is almost one dimensional along the c crystallographic axis, whereas under compression, all three unit cell axes participate, albeit unequally.

Early chemistry in hot and dense nitromethane: Molecular dynamics simulations

We report density functional molecular dynamic simulations to determine the early chemical events of hot (T=3000 K) and dense (rho=1.97 g/cm(3), V/V(0)=0.68) nitromethane (CH(3)NO(2)). The first step in the decomposition process is an intermolecular proton abstraction mechanism that leads to the formation of CH(3)NO(2)H(+) and the aci ion H(2)CNO(2) (-). This event is also confirmed to occur in a fast annealing simulation to a final temperature of 4000 K at rho=2.20 g/cm(3). An intramolecular hydrogen transfer that transforms nitromethane into the aci acid form, CH(2)NO(2)H, accompanies this event. To our knowledge, this is the first confirmation of chemical reactivity with bond selectivity for an energetic material near the Chapman-Jouget state of the fully reacted material. We also report the decomposition mechanism followed up to the formation of H(2)O as the first stable product. We note that similarities in the global features of reactants, intermediates, and products of the reacting fluid seem to indicate a threshold for similar chemistry in the range of high densities and temperatures reported herein.

Structure, dynamics, and the electronic absorption of benzene-argon clusters

We present a new method for calculating cluster absorption spectra using classical molecular dynamics and simulated annealing techniques. We then apply this method to benzene–Ar clusters. Cluster absorption spectra are shown to be dominated by an inhomogeneous distribution of isomer absorptions. The absorption spectrum of each isomer, however, results from the interplay of structure, fluctuations, and dynamics. We find that accompanying the solid to liquid transition, there is a spectroscopic transition from a periodic to a decaying autocorrelation function of the electronic energy gap. Benzene–Ar clusters are found to undergo transitions from a solid to a 2D liquid to a 3D liquid as the number of Ar atoms is increased from 1 to 21 at 20 K.

Synthesis of glycine-containing complexes in impacts of comets on early Earth

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.

Ab initio simulation of the equation of state and kinetics of shocked water

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 H(2)O <--> 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.

An accurate equation of state for the exponential-6 fluid applied to dense supercritical nitrogen

The exponential-6 potential model is widely used in fluid equation of state studies. We have developed an accurate and efficient complete equation of state for the exponential-6 fluid based on HMSA integral equation theory and Monte Carlo calculations. Our equation of state has average fractional error of 0.2% in pV/NkBT and 0.3% in the excess energy Uex/NkBT. This is a substantial improvement in accuracy over perturbation methods, which are typically used in treatments of dense fluid equations of state. We have applied our equation of state to the problem of dense supercritical N2. We find that we are able to accurately reproduce a wide range of material properties with our model, over a range 0.01⩽P⩽100 GPa and 298⩽T⩽15 000 K.