Session N42: Focus Session: Simulations of Matter at Extreme Conditions I

Shock-Induced Chemical Reactions of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) have been found in the atmospheres of Jupiter and Titan, and also in meteorites, interplanetary dust, and circumstellar graphite grains. The ubiquity of these complex organic structures and their stability under extreme conditions make them a significant factor in discussions of brebiotic chemistry in the solar system. To study the shock-induced chemistry of PAHs under conditions appropriate for astrophysical impacts, molecular dynamics simulations have been carried out for solid naphthalene and anthracene using a reactive empirical potential. The major reaction channels for these two closely related compounds were found to be substantially different. Product distributions were also found to depend strongly on the orientation of the PAH crystal relative to the shock propagation direction.   

Interatomic bond-order potentials for atomistic simulations of materials at extreme conditions

Molecular dynamics simulations provide an excellent opportunity to address fundamental physics and chemistry of materials at extreme conditions. However, the results of MD modeling can only be as reliable as the ability of the interatomic potentials to properly describe a variety of chemical effects including bond-breaking and bond-making. Our recent MD simulations of shock compression of covalently bonded materials such as diamond and silicon using REBO interatomic potential for diamond and EDIP potential for Si showed that the properties of C and Si systems at large pressures and temperatures are not well described in spite of the fact that the near equilibrium properties of both diamond and silicon are well reproduced. We present new results on development of analytic bond-order potentials (BOPs) for covalently bonded materials at extreme conditions. These BOPs are derived using the powerful concepts of moments of density of states, Green's function and Lanczos recursion, applied within the two-center, orthogonal tight-binding bond representation of electronic structure. Importantly, they include explicit analytic expressions for both the $\sigma $ and $\pi $ bonds. We will describe details of BOP construction including devising a first-principle database of fundamental materials properties, its fitting by the tight-binding (TB) model, and devising the analytic BOPs using the direct link between TB and analytic BOPs via the bond orders. Validation of analytic BOPs by comparison with first-principles high-pressure data will also be discussed.   

First-principles modeling of energetic materials

The prediction of properties of energetic materials using atomic-scale simulation techniques is one of the promising areas of energetic materials (EM) research. One of the challenges is to understand the initial response of EM to shock loading based on fundamental atomic-scale properties of EM crystals. We report the results of first-principles density-functional calculations of static and thermodynamic properties of PETN, HMX and RDX molecular crystals including properties of different crystalline phases and their equations of states (EOS). The EOS are extended beyond simple isotropic constitutive relationships to include materials response upon uniaxial compressions and high pressures up to 100 GPa. The predictions of the theory are compared with recent experimental results.   

Direct Simulation of Detonations: Applications to the H$_{2}$-Cl$_{2}$ System

Earlier simulations in our laboratory showed that ultrafast detonations having steady-state velocities greater than predicted by the Chapman-Jouguet (CJ) and the Zeldovich-von Neumann-D\"{o}ring (ZND) theories could be produced by very fast model reactions. In this paper we will report matching studies incorporating a realistic treatment of the reaction H$_{2}$ + Cl$_{2}\to $ 2 HCl reacting by the Nernst chain reaction mechanism with ignition, propagation and termination steps along with the inclusion of rotational and vibrational degrees of freedom for diatomic species and the realistic treatment of energy exchanges among all species. The H$_{2}$-Cl$_{2}$ system is the prototypical system for studying detonations both experimentally and theoretically, and is an ideal candidate for investigation. Our simulations are made using Bird's direct simulation Monte Carlo method which produces the full details of the coupled gas-dynamic and reaction effects as well as temperature, velocity, density, pressure, and species profiles for the detonation waves. By comparing predictions with available experimental measurements for the system, we will be able to predict the likelihood that ultrafast detonations can be observed for the H$_{2}$-Cl$_{2}$ reaction.   

Molecular Dynamics Simulations of Thermal Induced Chemistry in TATB

Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers$^{\dag }$ at CalTech and has already shown promise in predicting the chemistry in small samples of RDX under either shock compression or intense heat. Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. We will show results of 100,000-particle simulations at several temperatures, carried out with the massively parallel GRASP MD software developed at Sandia National Lab. Finally, we will compare the reactions and reaction timescales with those of RDX and HMX. $^{\dag }$ A. C. T. Van Duin, \textit{et al}, \textit{J. Phys. Chem. A}, \textbf{1005}, 9396 (2001).   

Decomposition of Energetic Materials at Extreme Conditions

Detailed description of chemical reaction mechanisms of solid energetic materials at high-density and temperatures is essential for understanding events that occur at the reactive front of these materials, and for the subsequent development of predictive models of materials properties. In this talk, we will report the results of our ongoing ab initio based molecular dynamic simulation of the chemistry of TATB, at density of 2.9 g/cm$^{3}$ and temperature of 1500K, and at density of 2.87 g/cm$^{3}$ and temperature of 2500 K. These conditions are similar to those experienced at the CJ and von Neumann spike Following the dynamics for a time scale of up to forty picoseconds allows the construction of approximate rate laws for typical products such as H$_{2}$O, N$_{2}$, CO, and CO$_{2}$. The approximate reaction rates obtained for these products at the CJ state will be compared to those obtained recently for HMX at similar conditions. This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-Eng-48.   

Transitions between strong shock and elastic-plastic shock regimes in tin and Lennard-Jonesium.

We have recently developed the Continuous Hugoniot Method as an efficient numerical approach to study the front dynamics of steady shocks. This method produces a continuous set of shock states as a function of the shock strength driving parameter. We use our method to investigate the shock states at and near the transition between the strong shock and elastic-plastic shock regimes in both single-crystal tin and Lennard-Jonesium along the $\langle 100 \rangle$ directions.   

Multi-scale molecular dynamics simulations of steady shock waves in Lennard-Jones and nitromethane

We compare spatial profiles of steady shock waves using our multi-scale simulation technique (Phys. Rev. Lett. 90, 235503 (2003)) and direct simulation techniques and find good agreement. Multi-scale simulations of shocked amorphous Lennard-Jones are in good agreement with NEMD simulations and multi-scale simulations of shock waves in analytical equations of state of explosives are in good agreement with hydrodynamic simulations. In both cases, agreement improves with distance behind the shock front. We have applied the multi-scale technique to the study of chemically reactive shock waves in condensed nitromethane (CH3NO2) using the density-functional tight-binding (DFTB) method. We study shock waves with speeds ranging from 5.5 km/s to 8 km/s for durations up to 0.5 ns behind the shock front. We believe these are the longest duration tight-binding simulations of shocked matter ever performed.   

First-principles approach to reactivity in the presence of shockwave

Gaining insight into the mechanisms leading to detonation in energetic materials within a planar shock geometry had been thought to be computationally prohibitive. The use of the Multiscale Shock Method (MSSM) of Reed et. al has opened the possibilities to study the chemistry of complex molecular systems undergoing uniaxial shock compression using Kohn-Sham density functional theory (KS-DFT). Here, we present results of nitromethane under various levels of shock loading and reveal the chemical mechanisms underlying the detonation process. We will also discuss an alternative non-Hamiltonian formulation of the MSSM. The aforementioned formulation is present in both CPMD and CP2K software packages. We also will compare and contrast different formulations of KS-DFT (both plane- wave and hybrid schemes) and discuss future of scaling the MSSM method to tera- scale platforms such as Blue Gene/L.   

Finding the structure of phosphorus in phase IV by the first-principles calculation

Phosphorus in phase IV (P-IV) had been unclear since first experimental report. Using the metadynamics combined with the first-principles calculation, we obtained a new structure. The structure is a monoclinic of sc: a=c=4.22{\AA}, b=4.15{\AA}, $\beta$=97.76$^{\circ}$ and a modulation is observed along the b-axis. We noted this modulated pattern and, for modulated structures having other wave-lengths, compared the x-ray diffraction patterns of these structures with experimental one. As a result, limited to the case of commensurate patterns because of the periodic boundary condition for the calculations, the structure whose period is 4 times as long as that of the non-modulated structure is the most compatible for the experimental result. For this pattern, calculating the enthalpy for pressure, it is lower than both sc and sh in the range from 118 GPa to 128 GPa. Recently Akahama \textit{et al.} have determined the structure of P-IV, which is rather close to our structure. Our calculation shows the transition from sc to P-IV was the first order phase transition.   

First Principles Phonon and Elasticity Computations for Iron under extreme conditions

We performed linear-response Linear-Muffin-Tin-Orbital (LMTO) and particle-in-cell (PIC) model calculations to understand and predict the lattice dynamical, thermal equation of state and elastic properties of bcc, fcc, and hcp iron as functions of pressure and temperature. The phonon dispersion and phonon density of states have been calculated at different volumes and show good agreement with experiment. We derived the Helmholtz free energy based on both the linear response LMTO and PIC calculations, and found that the calculated geometric mean phonon frequencies and free energies from these two different methods agree well under pressure, in contradiction to an earlier calculation. We performed detailed investigations on the behavior of elastic constants and various thermal equation of state parameters, including the bulk modulus, the thermal expansion coefficient, the Gr\"{u}neisen ratio, and the heat capacity as functions of temperature and pressure. We made detailed comparison with experiment and earlier theoretical calculations. We do not find the large change in c/a axial change with T. Sound velocities at extreme conditions have also been examined. These first-principles data provide important information to understand shock dynamics and other interesting phenomena under extreme conditions.   

Ab initio studies of magnetism at extreme volume and shape deformation

Magnetic solids constitute a basis of many technologically important materials, however, very little is known how their magnetic behavior changes when a high-strain deformation is applied (as it is, for example, in heavily deformed regions of extended defects, such as grain boundaries, dislocation cores, crack tips etc.). In the present talk, we report on magnetic behavior of iron, nickel, FeCo, Ni$_{3}$Al and Fe$_{3}$Al at the extreme volume as well as tetragonal and trigonal deformation. The total energies are calculated by spin-polarized full-potential LAPW method and are displayed in contour plots as functions of tetragonal or trigonal distortion $c/a$ and volume; borderlines between various magnetic phases are shown. Stability of tetragonal magnetic phases of $\gamma $-Fe is discussed. In case of Fe, Ni and FeCo, the calculated phase boundaries are used to predict the lattice parameters and magnetic states of overlayers from these materials on various (001) substrates. Whereas magnetism does not play an important role in stabilization of the L1$_{2}$ structure in Ni$_{3}$Al, the magnetic effects in Fe and Fe$_{3}$Al are vital.   

Electronic Structure of Actinides under Pressure

The series of heavy radioactive elements known as the actinides all have similar elemental properties. However, when the volume per atom in the condensed phase is illustrated as a function of atomic number, perhaps the most dramatic anomaly in the periodic table becomes apparent. The atomic volume of americium is almost 50{\%} larger than it is for the preceding element plutonium. For the element after americium, curium, the atomic volume is very close to that of americium. The same holds also for the next elements berkelium and californium. Accordingly from americium and onwards the actinides behave very similar to the corresponding rare-earth elements - a second lanthanide series of metallic elements can be identified. This view is strongly supported by the fact that all these elements adopt the dhcp structure, a structure typical for the lanthanides. The reason for this behavior is found in the behavior of the 5f electrons. For the earlier actinides, up to and including plutonium, the 5f electrons form metallic states and contribute most significantly to the bonding. In Np and Pu they even dominate the bonding, while all of a sudden they become localized in Am, very much like the 4f electrons in the lanthanide series, and contribute no longer to the cohesion. This withdrawal of 5f bonding gives rise to the large volume expansion between plutonium and americium. This difference between the light and heavy actinide suggests that it would be most worthwhile to strongly compress the transplutonium elements, thereby forcing the individual 5f electron wave functions into strong contact with each other (overlap). Recently high pressure experiments have been performed for americium and curium and dramatic crystal structure changes have been observed. These results and other high pressure data will be discussed in relation to the basic electronic structure of these elements.