Session W42: Focus Session: Simulations of Matter at Extreme Conditions III

Ab Initio Studies of High Pressure States of Crystalline Nitromethane.

We have calculated the mechanical compression curve for solid nitromethane with the ab-initio periodic structure code CRYSTAL using both Hartree-Fock and Density Functional Methods. In addition, calculations with both 6-21G and 6-31G** basis sets were performed and the effect of basis set superposition error was estimated using the counterpoise method. In each calculation the internal atomic coordinates and the crystal lattice parameters were relaxed at constant unit cell volume to the minimum energy configuration. The 6-31G** basis set was optimized by scaling the outer valence and polarization orbitals. It was found that Hartree-Fock calculations with a 6-21G basis set, uncorrected for basis set superposition error, gave the best agreement with experiment. These results may be due to the cancellation of basis set superposition error with dispersion force errors. While this result may be accidental, it appears that it extends to a number of other energetic organic molecular crystals, including beta HMX, PETN, and 1,1-diamino-2,2-dinitroethylene.   

Atomistic Studies of Plastic Deformation and Dissipation in Crystalline HMX

We are using large scale molecular dynamics simulations of crystalline octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) to better understand the dominant fundamental mechanisms of inelastic deformation and other dissipative processes in anisotropic organic molecular crystals. A fully flexible force field (Smith, G. D. and Bharadwaj, R. K.;\textit{ J. Phys. Chem. B} 1999, 103, 3570) used in numerous preceding studies is used without modification in the present work. Our results, based on the results of simulations containing 25,000-250,000 molecules, indicate a large degree of directional anisotropy in response to compression, for both quasi-static and shock loading. Plastic deformation is observed for some loading directions whereas solid-solid phase transitions are observed for others. The emphasis of the present talk will be identifying and characterizing detailed molecular mechanisms and rate dependencies in those cases for which dislocation-induced plasticity occurs.   

First-principles Study of Shock Compressed Carbon

The phase diagram of carbon at high pressures and temperatures is of scientific interest to material science, geology and astrophysics. Major issues include the liquid-liquid phase transition, the melting curve of graphite and diamond, the nature of the liquid state and the nature of carbon in the interior of Uranus and Neptune. Strong shock waves generated by lasers, and even nuclear explosions have been used to study carbon at these extreme conditions. Because it is often difficult to replicate these shock-wave experiments, first-principles electronic structure calculations can play a prominent role in verifying, guiding, and interpreting these experiments. We report DFT results for the diamond Hugoniot.   

Molecular dynamics simulation of shock compression of silicon

Shock compression of condensed matter is a fascinating scientific field that provides an excellent opportunity to probe the fundamental physics and chemistry of matter at extreme pressures and temperatures. In spite of substantial theoretical and experimental efforts, a full understanding of shock-induced elastic and plastic responses and polymorphic phase transitions is still far from complete. These phenomena often occur at the nanometer size and picosecond time scales, which makes molecular dynamics simulations an ideal tool for exploring nanoscale mechanisms of shock induced processes such as chemical reactions and phase transitions. We report the results of a molecular dynamics simulation of shock wave propagation in silicon in the [100], [110], and [111] directions obtained using a classical interatomic potential. Several regimes of materials response are classified as a function of shock wave intensity and crystalline orientation of shock wave propagation using calculated shock Hugoniot. The shock induced chemistry and shock wave splitting are discussed in relation to recent experimental results [1] that indicate an anomalous elastic response of the lattice at high compression ratios. [1] A. Loveridge-Smith, Phys. Rev. Let. \textbf{86}, 2349 (2001).   

Molecular Dynamics Studies of Dynamical High-Pressure Phase Transitions in Rare-Gas Solids

The phase diagram of pair potential models of rare-gases was studied with respect to the effect of the choice of potential on the nature of the phase diagram. In particular the existence of a high-pressure bcc phase is shown to be potential sensitive. We show using molecular dynamics that the fcc-bcc phase transition cannot be reproduced with the Lennard-Jones (12-6) pair potential, though it is reproduced with the Buckingham pair potential. We propose a simple analytical technique, based on the Einstein theory of a harmonic solid, for predicting an fcc-bcc phase transition in a given system. Using the atomic volume and the pair potential as input, we were able to predict the transition temperature. These findings agree with an earlier work by A. B. Belonoshko et al., Phys. Rev. Lett. 87, 165505 (2001). Additionally, shock wave simulations of several model systems were conducted. The structure of shock wave in this model was examined as a function of shock strength and the existence of a dynamic phase transition was explored.   

Interfacial instabilities and structure during high velocity sliding

Interfacial sliding under high pressure loading at high velocities (0 $<$ v $<$ 1 km/s) results in a variety of mesosc ale phenomena at extreme strain rates. For ductile metal interfacial pairs, these include nano- and mesoscale dynami c strucutral transitions, local melting and amorphization, material mixing, and localization of plastic deformation. We illustrate these phenomena with large scale NonEquilibrium Molecular Dynamics (NEMD) simulations for Cu/Ag, Ta/Al, and Al/Al interfaces. These suggest universal behavior in sliding velocity for the frictional force and a scaling form for the frictional force vs. velocity at high velocities which will be discussed.   

Simulations of Rapid Solidification in Metals at High Pressure

Although computer simulation has played a central role in the study of nucleation and growth since the earliest molecular dynamics simulations almost 50 years ago, confusion surrounding the effect of finite size on such simulations have limited their applicability. Modeling molten tantalum in systems ranging from 64,000 to 131,072,000 atoms on the BlueGene/L computer, I will discuss the first atomistic simulations of solidification that demonstrate independence from finite size effects during the entire nucleation and growth process, up to the onset of coarsening. Using both our new results and historical data, we show that the observed maximal grain sizes for systems smaller than about 8,000,000 atoms are functions of the simulation size, following the predictions of finite size scaling theory. For larger simulations, a crossover from finite size scaling to more physical size-independent behavior is observed.   

Properties of molten sodium under pressure from first principles theory.

Recent measurements of the melting curve of sodium [1] have found a sharp decline in the melting temperatures from 1000 to 300 K in the pressure range from 30 to 120 GPa. In this study, we investigate the stability and structural properties of solid and liquid sodium at high pressure and temperature using first principles molecular dynamics. The experimental melting curve is reproduced from 0 to 120 GPa. The local structure of the liquid is found to be strongly correlated to the multiple finite temperature crystalline phases of sodium. Based on a quantitative analysis of the structural and electronic properties of the solid and liquid phases, we propose an explanation for the unusual melting curve and a new perspective on the phase diagram of sodium. [1] Gregoryantz et al., Phys. Rev. Lett. 94, 185502 (2005).   

Melting and phase stability of high-density beryllium

First-principles Molecular Dynamics calculations have been performed to determine the liquid vs. solid phase boundary for beryllium up to 250 GPa. Shock Hugoniot curves have been calculated for both solid and liquid beryllium in this range of pressures and temperatures to determine the shock melting onset conditions and pressure range of liquid-solid coexistence. The results of these simulations also provide insights on the problem of relative stability of various crystalline forms of beryllium at high temperature. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W- 7405-Eng-48.   

Ab initio simulation of intense short-pulse laser irradiation of metals and semi-conductors

The effect of intense ultra-laser irradiation on crystal stability is not completely elucidated. Ultrashort laser pulses heat electrons to a very high temperature and leave the lattice relatively cool since the heat capacity of electrons is much smaller than that of lattice. This non-equilibrium system can be described as two subequilibrium systems : the hot electrons and a cold lattice. We studied the effect of this intense electronic excitations on the interatomic forces and the possible melting of the underlying lattice for a semi-conductor (Si) and two metals (Al and Au). We used {\it ab initio} linear response to compute the phonon spectrum in the Density Functional Theory framework for several electronic temperatures ranging from 1 to 6 eV. We found that semi-conductors and metals behave in an opposite ways when increasing electronic temperature. Phonon instability appears in silicon at a electronic temperature of $1.5\;\rm{eV}$ inducing the melting of the lattice. Gold samples become more stable. The Debye temperature was deduced from the phonon spectrum and using the Linderman criterion, we showed that gold undergoes a sharp increase of its melting temperature under intense laser irradiation. The same effect is observed for aluminium.   

Nonequilibrium Dynamics of Ultracold Neutral Plasmas

In a number of recent experiments ultracold plasmas (UNPs) have been produced by photoionizing laser-cooled atomic ensembles [1]. Their very low initial kinetic energies suggest that they are created deeply in the strongly correlated regime. Moreover, UNPs are produced far from equilibrium, leading to a complex relaxation dynamics. We present a hybrid-molecular dynamics approach [2], to describe the long-time plasma evolution while fully taking into account the strongly correlated character of the ionic motion. We demonstrate that the method yields an accurate description of recent measurements [2,3] and allows to address problems beyond present experimental capabilities [3]. It turns out that under the conditions in UNPs the commonly applied Bogoliubov assumption about a hierarchy of relaxation timescale becomes invalid, resulting in an unusual relaxation dynamics connected with a wave-like temperature evolution and an ultimate relaxation to a non-equilibrium undercorrelated state.\\ (1) Y.C. Chen et al., Phys. Rev. Lett. 93, 265003 (2004).\\ (2) T. Pohl, T. Pattard and J.M. Rost, Phys. Rev. A 70, 033416 (2004).\\ (3) T. Pohl, T. Pattard and J.M. Rost, Phys. Rev. Lett. 94, 205003 (2005); Phys. Rev. Lett. 92, 205003 (2004).   

Shock-Induced Polarization in Distilled Water

This study is aimed at developing a theoretical model to describe shock-induced polarization in water. The model is based on the notion that polar water molecules tend to align in the shock front due to inertial and stress forces. Analytical formulas for calculation of the shock-induced polarization charge, potential generated by this charge, and accompanied polarization current produced by the shock wave are derived. A comparison with experimental curves for polarization currents suggests that two factors contribute into the measured polarization signal: change of the polarization charge once the wave front enters the sample and change of the sample capacity while the front is progressing across the sample. Good agreement with experimental data on polarization in distilled water leads us to believe that the results obtained bring about a better understanding of mechanisms of shock induced polarization in liquids containing polar molecules.   

Quantum Dynamics of Energy Transfer under Shock Conditions

Classical molecular dynamics (MD) simulations predict efficient energy transfer from translational to vibrational modes near shock fronts in molecular solids. The validity of the classical description of collisional energy transfer under shock conditions has not been tested for extended systems. In this research effort, quantum mechanical (QM) simulations are used to study energy transfer in a system consisting of three collinear diatomic molecules and a stationary wall. A fast-moving projectile diatom collides with its neighbor initiating a collision cascade. The multiplicity of collisions precludes \textit{a priori} prediction of the detailed collision dynamics. The time dependence of the six-degrees-of-freedom wave function is determined using QM time-dependent wave packet methods. Intra- and inter-molecular interactions are described using nearest-neighbor potentials. Probabilities for vibrational excitation and bond rearrangement are predicted as a function of the collision energy of the projectile for differing interaction potentials and atomic masses.