Scalable Methods for Electronic Excitations and Optical Responses of Nanstructures: Mathematics to Algorithms to Observables

This multi-investigator project was concerned with the development and application of new methods and computer codes that would allow realistic modeling of nanosystems. Carter's part in this team effort involved two method/algorithm/code development projects during the first 14 months of this grant. Carter's group has been advancing theory and applications of the orbital-free density functional theory (OF-DFT), the only DFT method that exhibits linear scaling for metals. Such a method offers the possibility of simulating large numbers of atoms with quantum mechanics, such that properties of metallic nanostructures (e.g. nanowires of realistic dimensions) could be investigated. In addition, her group has been developing and applying an embedded correlated wavefunction theory for treating localized excited states in condensed matter (including metals). The application of interest here is spin manipulation at the nanoscale, i.e., spintronics, in which local electron excitations interact with the surrounding material. Her embedded correlation method is ideal for studying such problems

Sponsored by ASABE Galt House

Abstract. Several studies were conducted by Forest Service researchers and University and Industrial collaborators that investigated the potential for lessening soil surface disturbances and compaction in forest operations through modifications of machine components or harvest systems. Specific machine modifications included change in tire size, use of dual tire systems, reduction of tire inflation pressures, reductions in load size and ground pressure. Soil surface disturbances were most evident in sites with high soil moisture content that were lessened by lowering tire inflation pressures or using a dual tire configuration. Traffic intensity increased rutting potential of harvest sites, especially with the use of narrow tires. Traffic intensities varied spatially and in intensity in clear cut harvest operations with intensities that ranged between none to 100 or more. Soil physical properties responded to choice of tire size and inflation pressure with narrower tires and/or higher inflation pressures associated with increased soil compaction. Soil disturbance data collected in three clear cut operations in Alabama indicated no differences among the operations by location, but soil response varied depending on site properties. Soil physical properties did not necessarily reflect the intensity of soil disturbance

Prediction of a Highly Activated State of CO Adsorbed on an Al/Fe(100) Bimetallic

Using periodic slab density functional theory, we investigate CO adsorption, diffusion, and dissociation energetics on a monolayer of Al covering Fe(100) [Al/Fe(100)]. We predict a weakly chemisorbed state of CO to exist on Al/Fe(100), with CO adsorbing on the 4-fold hollow site in a very tilted fashion. This state is predicted to have an extremely low CO stretching frequency of only 883 cm-1, indicating a dramatically weakened CO bond relative to gaseous CO, even though the molecule is predicted to bind to Al/Fe(100) quite weakly. We predict that dissociation of CO starting from this weakly adsorbed state has a barrier of only ∼0.35 eV, which is ∼0.70 eV lower than that on Fe(100). To understand how the underlying substrate changes the electronic properties of the supported Al monolayer, we compare CO adsorption on Al/Fe(100) to its adsorption on analogous pure Al(100) surfaces. This highly activated yet weakly bound state of CO on Al/Fe(100) suggests that Al/Fe(100) could be an effective low-temperature bimetallic catalyst in reducing environments. 1

Structure of and ion segregation to an alumina grain boundary: implications for growth and creep

Using periodic density-functional theory (DFT), we investigated the structure and cohesive properties of the �-alumina �11 tilt grain boundary, with and without segregated elements, as a model for the thermally grown oxide in jet engine thermal barrier coatings. We identified a new low-energy structure different from what was proposed previously based on electron microscopy and classical potential simulations. We explored the structure and energy landscape at the grain boundary for segregated Al, O, and early transition metals (TMs) Y and Hf. We predict that the TMs preferentially adsorb at the same sites as Al, while some adsites favored by O remain unblocked by TMs. All segregated atoms have a limited effect on grain boundary adhesion, suggesting that adhesion energies alone cannot be used for predictions of creep inhibition. These findings provide some new insights into how TM dopants affect alumina growth and creep kinetics. I

2005 American Conference on Theoretical Chemistry

The materials uploaded are meant to serve as final report on the funds provided by DOE-BES to help sponsor the 2005 American Conference on Theoretical Chemistry

Scalable Methods for Electronic Excitations and Optical Responses of Nanostructures: Mathematics to Algorithms to Observables

Kohn-Sham density functional theory (DFT) is a powerful, well-established tool for the study of condensed phase electronic structure. However, there are still a number of situations where its applicability is limited. The basic theme of our research is the development of first principles electronic structure approaches for condensed matter that goes beyond what can currently be done with standard implementations ofKohn-Sham DFT. Our efforts to this end have focused on two classes or' methods. The first addresses the well-lmown inability of DFT to handle strong, many-body electron correlation effects. Our approach is a DFT -based embedding theory, to treat localized features (e.g. impurity, adsorbate, vacancy, etc.) embedded in a periodic, metallic crystal. A description for the embedded region is provided by explicitly correlated, ab initio wave function methods. DFT, as a fo1n1ally ground state theory, does not give a good description of excited states; an additional feature of our approach is the ability to obtain excitations localized in this region. We apply our method to a first-principles study of the adsorption of a single magnetic Co ada tom on non-magnetic Cu( 111 ), a known Kondo system whose behavior is governed by strong electron correlation. � The second class of methods that we are developing is an orbital-free density functional theory (OFDFT), which addresses the speed limitations ofKohn-Sham DFT. OFDFT is a powerful, O(N) scaling method for electronic structure calculations. Unlike Kohn-Sham DFT, OFDFT goes back to the original Hohenberg-Kohn idea of directly optimizing an energy functional which is an explicit functional of the density, without invoking an orbital description. This eliminates the need to manipulate orbitals, which leads to O(N{sup 3}) scaling in the Kahn-Sham approach. The speed of OFDFT allows direct electronic structure calculations on large systems on the order of thousands to tens of thousands of atoms, an expensive feat within Kohn-Sham. Due to our incomplete knowledge of the exact, universal energy density functional, this speedup comes at the cost of some accuracy with respect to Kohn-Sham methods. However, OFDFT has been shown to be remarkably accurate with respect to Kohn-Sham when used in the study of nearly-free-electron-like metals, e.g., AI, for which good density functionals have been derived. Examples of past applications of OFDFT include the prediction of properties of bulk crystals, surfaces, vacancies, vacancy clusters, nanoclusters, and dislocations, as well as OFDFT -based multiscale simulations of nanoindentation in AI and Al-Mg alloys

Universal binding-energy relation for crystals that accounts for surface relaxation

We present a universal relation for crack surface cohesion including surface relaxation. Specifically, we analyze how N atomic planes respond to an opening displacement at its boundary, producing structurally relaxed surfaces. Via density-functional theory, we verify universality for metals (Al), ceramics (α−Al_2O_3), and semiconductors (Si). When the energy and opening displacement are scaled appropriately with respect to N, the uniaxial elastic constant, the relaxed surface energy, and the equilibrium interlayer spacing, all energy-displacement curves collapse onto a single universal curve