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We have revealed, and resolved, an apparent inability of density functional theory, within the local density and generalized gradient approximations, to describe vacancies in Al accurately and consistently. The shortcoming is due to electron correlation effects near electronic edges and we show how to correct for them. We find that the divacancy in Al is energetically unstable and we show that anharmonic atomic vibrations explain the non-Arrhenius temperature dependence of the vacancy concentration.

The isotope effect in quantum diffusion of H on Ni(001) is thoroughly treated using the path-centroid method for the transition rate combined with a potential fitted to first-principles calculations. Around room temperature, our results are in quantitative agreement with experimental results. At low temperatures, we find a transition to temperature-independent diffusion, but no signs of an anomalous isotope dependence.

We have computed the diffusion constant for hydrogen and deuterium on Ni(001) using the quantum version of transition-state theory formulated in terms of the transition rate for the path centroid. The embedded-atom method was used for the potential. We find a transition beween thermally activated diffusion at high temperatures and almost temperature-independent diffusion at lower temperatures. We interpret the transition as being from semiclassical motion over the barrier to quantum tunneling through the barrier. One-dimensional model calculations suggest that the distinct transition is due to the shape of the potential barrier along the surface. The calculated values for the diffusion constant at high temperatures agree with the experimental results; however, the transition is located at a too low temperature (40 K) compared with experiments (100 K). For deuterium the transition occurs at approximately 25 K, which is in disagreement with the weak isotope effect found in the experiments.

An accurate equation of state (EOS) for polyethylene is required in order to model high energy density experiments for CH2 densities above 1 g/cc, temperatures above 1 eV, and pressures above 1 Mbar. Density Functional Theory (DFT) based molecular dynamics has been established as a method capable of yielding high fidelity results for many materials at a wide range of pressures and temperatures and has recently been applied to complex polymers such as polyethylene [1]. Using high density polyethylene as the reference state, we compute the principal Hugoniot to 350 GPa, compression isentrope, and several release isentropes from states on the principal Hugoniot. We also calculate the specific heat and the dissociation along the Hugoniot. Our simulation results are validated by comparing to experimental data [2, 3] and then used to construct a wide range EOS.

Organic polymers and nanocomposites are increasingly being subjected to extreme environments. Molecular-scale modeling of these materials offers insight into failure mechanisms and response. In previously published work, we used classical molecular dynamics (MD) and density functional theory (DFT) simulations to determine the principal shock Hugoniot for two hydrocarbon polymers, polyethylene (PE) and poly(4-methyl-1-pentene) (PMP). DFT was in excellent agreement with experiment, and one of four classical MD potentials, ReaxFF, was found to be suitable for studies up to 50 GPa. Here, we extend these results to include low-density polymer foams using NEMD techniques. We find good quantitative agreement with both experiment and hydrocode simulations. Further, we have measured local temperatures to investigate the formation of hot spots and polymer dissociation near foam voids.

Hydrocarbon foams are commonly used in high energy-density physics (HEDP) applications, for example as tamper and ablation materials for dynamic materials or inertial confinement fusion (ICF) experiments, and as such are subject to shock compression from tens to hundreds of GPa. Modeling of macro-molecular materials like hydrocarbon foams is challenging due to the heterogeneous character of the polymers and the complexity of voids and large-scale structure. Under shock conditions, these factors contribute to a relatively larger uncertainty of the post-shock state compared to that encountered for homogenous materials; therefore a quantitative understanding of foams under strong dynamic compression is sought. We use Sandia's ALEGRA-MHD code to simulate 3D mesoscale models of poly-(4-methyl-1-pentene) (PMP) foams. We devise models of the initial polymer-void structure of the foam and analyze the statistical properties of the initial and shocked states. We compare the simulations to multi-Mbar shock experiments conducted on Sandia's Z machine at various initial foam densities and flyer impact velocities. Scatter in the experimental data may be a consequence of the initial foam inhomogeneity. We compare the statistical properties of the simulations with the scatter in the experimental data.

Molecular dynamics simulation (MD) is an invaluable tool for studying problems sensitive to atomscale physics such as structural transitions, discontinuous interfaces, non-equilibrium dynamics, and elastic-plastic deformation. In order to apply this method to modeling of ramp-compression experiments, several challenges must be overcome: accuracy of interatomic potentials, length- and time-scales, and extraction of continuum quantities. We have completed a 3 year LDRD project with the goal of developing molecular dynamics simulation capabilities for modeling the response of materials to ramp compression. The techniques we have developed fall in to three categories (i) molecular dynamics methods (ii) interatomic potentials (iii) calculation of continuum variables. Highlights include the development of an accurate interatomic potential describing shock-melting of Beryllium, a scaling technique for modeling slow ramp compression experiments using fast ramp MD simulations, and a technique for extracting plastic strain from MD simulations. All of these methods have been implemented in Sandia's LAMMPS MD code, ensuring their widespread availability to dynamic materials research at Sandia and elsewhere.

We have determined the diffusion constant for H on Ni(100) using the path-centroid formula for the transition rate, the embedded atom method for the potential, and the numerical quantum Monte Carlo technique. We find a distinct transition from semiclassical motion over the barrier to quantum tunneling through the barrier, in accordance with recent experiments by Lin and Gomer. The diffusion constant at high temperatures agrees with the experimental results; the transition, however, is located at too low a temperature (40 K) compared with experiment (100 K)

The path-integral formulation of quantum mechanics is a suitable starting point for quantum simulation studies. We have considered H diffusion on the Ni (100)-surface at room temperature and below using the quantum Monte Carlo technique. The transition rate is determined using the path-centroid formulation and the simulations are based on a potential derived from recent first-principles calculations of the total energy. We find a marked change of the temperature dependence for the diffusion constant around 60 K, indicating that quantum tunneling between the ground states starts to dominate the diffusion process. The simulation results are compared with recent experimental findings.

Lithium is a prototypical simple metal at standard conditions which is well described within the nearly free electron model. However, by changing the density towards expanded or compressed states, the electrical conductivity shows strong and partly unexpected variations. We have performed quantum molecular dynamics simulations for fluid lithium for a wide range of densities and temperatures in order to derive the equation of state, the electrical conductivity, and information about structural and electronic changes along the expansion or compression. Based on these results, we can give a consistent description of the electrical conductivity from the nonmetallic expanded fluid up to the degenerate electron liquid at high densities.

Knowledge of the properties of water is essential for correctly describing the physics of shock waves in water as well as the behavior of giant planets. By using finite temperature density functional theory (DFT), we have investigated the structure and electronic conductivity of water across three phase transitions (molecular liquid/ ionic liquid/super-ionic/electronic liquid). There is a rapid transition to ionic conduction at 2000 K and 2 g/cm{sup 3} while electronic conduction dominates at temperatures above 6000 K. We predict that the fluid bordering the super-ionic phase is conducting above 4000 K and 100 GPa. Earlier work instead has the super-ionic phase bordering an insulating fluid, with a transition to metallic fluid not until 7000 K and 250 GPa. The tools and expertise developed during the project can be applied to other molecular systems, for example, methane, ammonia, and CH foam. We are now well positioned to treat also complex molecular systems in the HEDP regime of phase-space.

Warm dense matter is the region in phase space of density and temperature where the thermal, Fermi, and Coulomb energies are approximately equal. The lack of a dominating scale and physical behavior makes it challenging to model the physics to high fidelity. For Sandia, a fundamental understanding of the region is of importance because of the needs of our experimental HEDP programs for high fidelity descriptive and predictive modeling. We show that multi-scale simulations of macroscopic physical phenomena now have predictive capability also for difficult but ubiquitous materials such as stainless steel, a transition metal alloy.

We change the diffusion mechanism of adsorbed Ge-Si dimers on Si(001) using the electric field of a scanning tunneling microscope tip. By comparing the measured field dependence with first-principles calculations we conclude that, in negative field, i.e., when electrons are attracted towards the vacuum, the dimer diffuses as a unit, rotating as it translates, whereas, in positive field the dimer bond is substantially stretched at the transition state as it slides along the substrate. Furthermore, the active mechanism in positive fields facilitates intermixing of Ge in the Si lattice, whereas intermixing is suppressed in negative fields.

The predictive power of first-principles calculations of vacancy formation energies in metals (Pt. Pd. Mo) is improved by adding a correction for the intrinsic surface error in current implementations of density functional theory. The derived correction is given as a function of electron density; it can be explicitly applied to a wide range of systems. Density functional theory, contrary to claims in previous work, underestimates the vacancy formation energy when structural relaxation is included. This is the case whether using the local density- or the generalized gradient approximation for the exchange-correlation energy. With corrections for the intrinsic surface error we reach excellent agreement between calculated values using the two exchange-correlation functionals. Our final values for the three vacancy formation energies are 1.16, 1.70, and 2.98 eV for Pt, Pd, and Mo, respectively. The numbers are in good agreement with experimental data. We also calculate the harrier for vacancy diffusion in Pt to 1.43 eV.

Monovacancy diffusion alone dominates over diffusion due to divacancies and interstitials in Al for all temperatures up to the melting point. Deviations from a single Arrhenius dependence are due to anharmonicity. The conclusion is based on a combination of theoretical methods, from density functional theory to thermodynamic integration, without fitting to experimental data. The calculated diffusion rate agrees with experimental data over 11 orders of magnitude.

Hydrocarbon foams are versatile materials extensively used in high energy-density physics (HEDP) experiments. However, little data exist above 100 GPa, where knowledge of the behavior is particularly important for designing, analyzing, and optimizing HEDP experiments. The complex internal structure and properties of foam call for a multi-scale modeling effort validated by experimental data. We present results from experiments, classical molecular dynamics simulations, and mesoscale hydrodynamic modeling of poly(4-methyl-1-pentene) (PMP) foams under strong shock compression. Experiments conducted using the Z-machine at Sandia National Laboratories shock compress ∼0.300 g/cm3 density PMP foams to 185 GPa. Molecular dynamics (MD) simulations model shock compressed PMP foam and elucidate behavior of the heterogeneous foams at high pressures. The MD results show quantitative agreement with the experimental data, while providing additional information about local temperature and dissociation. Three-dimensional nm-scale hydrocode simulations of the foam show internal structure of pore collapse as well as provide detailed information on the foam state behind the shock front. Finally, the experimental and MD results are compared to continuum hydrodynamics simulations to assess a potential equation of state model for PMP foams to use in large scale hydrodynamics simulations.

We show that the AM05 functional [Armiento and Mattsson, Phys. Rev. B 72, 085108 (2005)] has the same excellent performance for solids as the hybrid density functionals tested in Paier et al. [J. Chem. Phys. 124, 154709 (2006); 125, 249901 (2006)]. This confirms the original finding that AM05 performs exceptionally well for solids and surfaces. Hartree-Fock hybrid calculations are typically an order of magnitude slower than local or semilocal density functionals such as AM05, which is of a regular semilocal generalized gradient approximation form. The performance of AM05 is on average found to be superior to selecting the best of local density approximation and PBE for each solid. By comparing data from several different electronic-structure codes, we have determined that the numerical errors in this study are equal to or smaller than the corresponding experimental uncertainties. (C) 2008 American Institute of Physics.

First-principles density-functional calculations of hydrogen adsorption on the Ni (001) surface have been performed in order to get a better understanding of adsorption and diffusion of hydrogen on metal surfaces. We find good agreement with experiments for the adsorption energy, binding distance, and barrier height for diffusion at room temperature, A model potential is fitted to the first-principles data points using the simulated annealing technique and the hydrogen band structure is derived by solving the three-dimensional Schrodinger equation, We find vibrational excitation energies slightly too high, with about 10%, compared with experiments and very narrow hydrogen bands. The experimentally observed absence of a pronounced isotope effect for hydrogen diffusion al low temperatures is discussed in terms of tunneling in a static three-dimensional potential.