We evaluate the ability of the embedded-atom method ͑EAM͒ potentials and the tight-binding ͑TB͒ method to predict reliably energies and stability of nonequilibrium structures by taking Cu as a model material. Two EAM potentials are used here. One is constructed in this work by using more fitting parameters than usual and including ab initio energies in the fitting database. The other potential was constructed previously using a traditional scheme. Excellent agreement is observed between ab initio, TB, and EAM results for the energies and stability of several nonequilibrium structures of Cu, as well as for energies along deformation paths between different structures. We conclude that not only TB calculations but also EAM potentials can be suitable for simulations in which correct energies and stability of different atomic configurations are essential, at least for Cu. The bcc, simple cubic, and diamond structures of Cu were identified as elastically unstable, while some other structures ͑e.g., hcp and 9R͒ are metastable. As an application of this analysis, nonequilibrium structures of epitaxial Cu films on ͑001͒-oriented fcc or bcc substrates are evaluated using a simple model and atomistic simulations with an EAM potential. In agreement with experimental data, the structure of the film can be either deformed fcc or deformed hcp. The bcc structure cannot be stabilized by epitaxial constraints.

The electrical conductivity of warm, dense aluminum plasmas and liquids is calculated using ab initio molecular dynamics and the Kubo-Greenwood formula. The density range extends from near solid to one-hundredth of solid density, and the temperature range extends from 6000 K to 30 000 K. This density and temperature range allows direct comparison with experimental results obtained with the tamped exploding wire technique.

A theoretical treatment based upon the quasi-chemical theory of solutions predicts the most probable number of water neighbors in the inner shell of a Li + ion in liquid water to be four. The instability of a six water molecule inner sphere complex relative to four-coordinated structures is confirmed by an 'ab initio' molecular dynamics calculation. A classical Monte Carlo simulation equilibrated 26 water molecules with a rigid six-coordinated Li(H 2 O) 6 + complex with periodic boundary conditions in aqueous solution. With that initial configuration for the molecular dynamics, the six-coordinated structure relaxed into four-coordinated arrangements within 112 fs and stabilized. This conclusion differs from prior interpretations of neutron and X-ray scattering results on aqueous solutions. * LA-UR-99-3360.

We use ab initio molecular dynamics as a basis for quasichemical theory evaluation of the free energy of water near conventional liquid thermodynamic states. The Perdew-Wang-91 (PW91), Perdew-Burke-Ernzerhof (PBE), and revised PBE (rPBE) functionals are employed. The oxygen radial density distribution using the rPBE functional is in reasonable agreement with current experiments, whereas the PW91 and PBE functionals predict a more structured oxygen radial density distribution. The diffusion coefficient with the rPBE functional is in reasonable accord with experiments. Using a maximum entropy procedure, we obtain x0 from the coordination number distribution xn for oxygen atoms having n neighbors. Likewise, we obtain p0 from pn, the probability of observing cavities of specified radius containing n water molecules. The probability x0 is a measure of the local chemical interactions and is central to the quasichemical theory of solutions. The probability p0, central to the theory of liquids, is a measure of the free energy required to open cavities of defined sizes in the solvent. Using these values and a reasonable model for electrostatic and dispersion effects, the hydration free energy of water in water at 314 K is calculated to be -5.1 kcal/mole with the rPBE functional, in encouraging agreement with the experimental value of -6.1 kcal/mole.

The hydroxide anion plays an essential role in many chemical and biochemical reactions. But a molecular-scale description of its hydration state, and hence also its transport, in water is currently controversial. A preeminent challenge in liquid-state physics is the understanding of aqueous phase chemical transformations on a molecular scale. Water undergoes limited autoprotolysis, which is enhanced in the presence of highly charged metal ions such as Be 2ϩ (1, 2). Understanding the hydration and transport of the autoprotolysis products, H ϩ and HO Ϫ , presents unique and interesting challenges for molecular-scale theories of solutions and for simulations. In this paper we focus on HO Ϫ (aq).Because H ϩ and HO Ϫ constitute the underlying aqueous matrix, it is not unreasonable to expect that their transport in water is different from the transport of other aqueous ions. This anomalous diffusion of the H ϩ (aq) and HO Ϫ (aq) has received extensive scrutiny over the years (for example, refs. 3-5), but recently ab initio molecular dynamics (AIMD) capabilities have evolved to provide new information on the solution condition and transport of these species. Over a similar period, the statistical mechanical theory of liquids (especially water) has also become more sophisticated (for example, ref. 6). These two approaches can be complementary, but in typical practice they remain imperfectly connected (but see refs. 2 and 7-11).In an initial AIMD study (12) Ϫ species was 2-3 ps, but statistical characterization was sketchy.Discussions of a transport mechanism for HO Ϫ (aq) typically focus on Agmon's (14, 15) extraction of an activation energy for hydroxide transport from the temperature dependence of the experimental mobilities. Near room temperature that empirical parameter is about 3 kcal͞mol, but it increases by roughly a factor of 2 for slightly lower temperatures. As a mechanical barrier this value, about 5-6 k B T, may be low enough to require some subtlety of interpretation (16); the observed temperature sensitivity of the activation energy, and particularly its increase with decreasing temperature, supports that possibility. We note that a standard inclusion of a tunneling correction would be expected to lead to a decrease of activation energy with decreasing temperature.Ref. 13 framed the consideration of HO Ϫ transport in terms of classical transition state theory and extracted an activation energy from the gas-phase study of Novoa et al. (17). Ref. 13 also considered the importance of tunneling in lowering the barrier for proton transfer by performing path integral calculations. Their combined value of 3

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We fit an empirical potential for silicon using the modified embedded atom (MEAM) functional form, which contains a nonlinear function of a sum of pairwise and three-body terms. The three-body term is similar to the Stillinger-Weber form. We parametrized our model using five cubic splines, each with 10 fitting parameters, and fitted the parameters to a large database using the force-matching method. Our model provides a reasonable description of energetics for all atomic coordinations, Z, from the dimer (Z = 1) to fcc and hcp (Z = 12). It accurately reproduces phonons and elastic constants, as well as point defect energetics. It also provides a good description of reconstruction energetics for both the 30° and 90° partial dislocations. Unlike previous models, our model accurately predicts formation energies and geometries of interstitial complexes - small clusters, interstitial-chain and planar {311} defects.

We have performed molecular dynamics simulations to obtain the internal energy and pressure of shockcompressed fluid deuterium at 24 separate ͑density temperature͒ points. Our calculations were performed using the generalized gradient approximation ͑GGA͒ in density-functional theory. We obtained a good fit to this simulation data with a thermodynamically consistent virial expansion. The single-shock Hugoniot derived from this equation of state is compared to previous theoretical and experimental results. We discuss several types of error inherent in the GGA, as they relate to the quality of our results.