Study of surfaces and interfaces using quantum chemistry techniques

There are a number of difficulties in elucidating the microscopic details of the electronic states at surfaces and interfaces. The first step should be to determine the structure at the surface or interface, but this is difficult experimentally even for the clean, ordered surface and extremely difficult for cases with impurity atoms (e.g., nonordered oxide layers). The theoretical study of such geometries and energy surfaces is the subject of quantum chemistry. We present a review of some of the theoretical techniques from quantum chemistry that are being applied to surfaces. The procedure consists of treating a finite piece of the surface or interface as a molecule. Ab initio calculations are then carried out on the molecule using the generalized valence bond (GVB) method (with additional configuration interaction), thereby incorporating the dominant many‐body effects. The reliability of these techniques is discussed by giving some examples from molecular chemistry and the surfaces of solids. The strengths and weaknesses of this approach are compared with more traditional band theory related methods and are illustrated with various examples

Excited Electron Dynamics Modeling of Warm Dense Matter

We present a model (the electron force field, or eFF) based on a simplified solution to the time-dependent Schrödinger equation that with a single approximate potential between nuclei and electrons correctly describes many phases relevant for warm dense hydrogen. Over a temperature range of 0 to 100 000 K and densities up to 1 g/cm^3, we find excellent agreement with experimental, path integral Monte Carlo, and linear mixing equations of state, as well as single-shock Hugoniot curves from shock compression experiments. In principle eFF should be applicable to other warm dense systems as well

The dynamics of highly excited electronic systems: Applications of the electron force field

Highly excited heterogeneous complex materials are essential elements of important processes, ranging from inertial confinement fusion to semiconductor device fabrication. Understanding the dynamics of these systems has been challenging because of the difficulty in extracting mechanistic information from either experiment or theory. We describe here the electron force field (eFF) approximation to quantum mechanics which provides a practical approach to simulating the dynamics of such systems. eFF includes all the normal electrostatic interactions between electrons and nuclei and the normal quantum mechanical description of kinetic energy for the electrons, but contains two severe approximations: first, the individual electrons are represented as floating Gaussian wave packets whose position and size respond instantaneously to various forces during the dynamics; and second, these wave packets are combined into a many-body wave function as a Hartree product without explicit antisymmetrization. The Pauli principle is accounted for by adding an extra spin-dependent term to the Hamiltonian. These approximations are a logical extension of existing approaches to simulate the dynamics of fermions, which we review. In this paper, we discuss the details of the equations of motion and potentials that form eFF, and evaluate the ability of eFF to describe ground-state systems containing covalent, ionic, multicenter, and/or metallic bonds. We also summarize two eFF calculations previously reported on electronically excited systems: (1) the thermodynamics of hydrogen compressed up to ten times liquid density and heated up to 200 000 K; and (2) the dynamics of Auger fragmentation in a diamond nanoparticle, where hundreds of electron volts of excitation energy are dissipated over tens of femtoseconds. These cases represent the first steps toward using eFF to model highly excited electronic processes in complex materials

High-temperature high-pressure phases of lithium from electron force field (eFF) quantum electron dynamics simulations

We recently developed the electron force field (eFF) method for practical nonadiabatic electron dynamics simulations of materials under extreme conditions and showed that it gave an excellent description of the shock thermodynamics of hydrogen from molecules to atoms to plasma, as well as the electron dynamics of the Auger decay in diamondoids following core electron ionization. Here we apply eFF to the shock thermodynamics of lithium metal, where we find two distinct consecutive phase changes that manifest themselves as a kink in the shock Hugoniot, previously observed experimentally, but not explained. Analyzing the atomic distribution functions, we establish that the first phase transition corresponds to (i) an fcc-to-cl16 phase transition that was observed previously in diamond anvil cell experiments at low temperature and (ii) a second phase transition that corresponds to the formation of a new amorphous phase (amor) of lithium that is distinct from normal molten lithium. The amorphous phase has enhanced valence electron-nucleus interactions due to localization of electrons into interstitial locations, along with a random connectivity distribution function. This indicates that eFF can characterize and compute the relative stability of states of matter under extreme conditions (e.g., warm dense matter)

Mechanisms of Auger-induced chemistry derived from wave packet dynamics

To understand how core ionization and subsequent Auger decay lead to bond breaking in large systems, we simulate the wave packet dynamics of electrons in the hydrogenated diamond nanoparticle C_(197)H_(112). We find that surface core ionizations cause emission of carbon fragments and protons through a direct Auger mechanism, whereas deeper core ionizations cause hydrides to be emitted from the surface via remote heating, consistent with results from photon-stimulated desorption experiments [Hoffman A, Laikhtman A, (2006) J Phys Condens Mater 18:S1517–S1546]. This demonstrates that it is feasible to study the chemistry of highly excited large-scale systems using simulation and analysis tools comparable in simplicity to those used for classical molecular dynamics

Mechanical properties of connected carbon nanorings via molecular dynamics simulation

Stable, carbon nanotori can be constructed from nanotubes. In theory, such rings could be used to fabricate networks that are extremely flexible and offer a high strength-to-density ratio. As a first step towards realizing such nanochains and nanomaile, the mechanical properties of connected carbon nanorings were investigated via molecular dynamics simulation. The Young's modulus, extensibility and tensile stength of nanorings were estimated under conditions that idealize the constraints of nanochains and nanomaile. The results indicate nanorings are stable under large tensile deformation. The calculated Young's modulus of nanorings was found increase with deformation from 19.43 GPa to 121.94 GPa (without any side constraints) and from 124.98 GPa to 1.56 TPa (with side constraints). The tensile strength of unconstrained and constrained nanorings is estimated to be 5.72 and 8.522 GPa, respectively. The maximum strain is approximately 39% (nanochains) and 25.2% (nanomaile), and these deformations are completely reversible

Development of Interatomic ReaxFF Potentials for Au-S-C-H Systems

We present fully reactive interatomic potentials for systems containing gold, sulfur, carbon, and hydrogen, employing the ReaxFF formalism. The potential is designed especially for simulating goldthiol systems and has been used for studying cluster deposition on self-assembled monolayers. Additionally, a large number of density functional theory calculations are reported, including molecules containing the aforementioned elements and adsorption energetics of molecules and atoms on gold

Molecular mechanics and molecular dynamics analysis of Drexler-Merkle gears and neon pump

Over the past two years at the Materials and Process Simulation Center, we have been developing simulation approaches for studying the molecular nanomachine designs pioneered by Drexler and Merkle. These nanomachine designs, such as planetary gears and neon pump, are described with atomistic details and involve up to 10 000 atoms. With the Dreiding and universal force fields, we have optimized the structures of the two planetary gear designs and the neon pump. At the Fourth Foresight conference, we reported rotational impulse dynamics studies of the first and second generation designs of planetary gears undergoing very high-frequency rotational motions. We will explore stability of these designs in the lower frequency regimes which require long time simulations. We will report the molecular mechanics and molecular dynamics simulations performed on these model systems. We explore the following modes in these studies: (1) impulse mode; (2) constant angular velocity - perpetual rotation; (3) constant torque - acceleration from rest

Initial Chemical Events in the Energetic Material RDX under Shock Loading: Role of Defects

We use the recently developed reactive force field ReaxFF with molecular dynamics (MD) to study the role of voids on the initial chemical events in the high-energy material RDX under shock loading. We find that for strong shocks (particles velocity of 3 km/s) very small gaps (2 nm) lead to important over-heating (~ 1000 K). This over-heating facilitates chemical reactions and leads to a larger production of small molecules (such as NO2, NO, OH) than in perfect crystals shocked with the same strength. The chemical reactions occur after the void has collapsed and the ejected material re-compressed rather than when hot molecules are ejected out of the downstream surface