Browsing by Subject "Density Functional Theory"

Now showing 1 - 8 of 8
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    Density functional theory: toward better understanding of complex systems in chemistry and physics
    (2014-06) Luo, Sijie
    Density functional theory (DFT) has become the workhorse of computational chemistry and physics in the past two decades. The continuous developments of high-quality exchange-correlation functionals (xcFs) have enabled chemists and physicists to study complex as well as large systems with high accuracy at low-to-moderate computational expense. Although a wide range of normal systems have been well understood by DFT, there are still complex ones presenting particular challenges where most commonly used xcFs have failed due to the complex nature of the system, lack of or difficulty to obtain reliable reference data, or the practical limitations of the Kohn-Sham DFT (KS-DFT) formulation.This thesis presents studies with various exchange-correlation functionals on a wide selection of complex systems in chemistry and solid-state physics, including large organic molecules, adsorption on metallic surfaces, transition states, as well as transition metal atoms, ions, and compounds, to (i) draw conclusions upon recommendations of xcFs for important practical applications; (ii) understand the root of errors to help design better xcFs or propose new theoretical schemes of DFT; (iii) explore the utility of noncollinear spin orbitals in KS-DFT for better description of multi-reference systems.
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    Developing New Kohn-Sham Density Functional for Molecules, Atoms, and Solids: New Methods and Applications
    (2016-06) Yu, Haoyu
    The accuracy of Kohn-Sham density functional theory depends on the exchange-correlation functional. Local functionals depending on only the density (ρ), density gradient (∇ρ), and possibly kinetic energy density (τ) have been popular because of their low cost and simplicity, but the most successful functionals for chemistry have involved nonlocal Hartree-Fock exchange (HFX). Based on the mathematical form of a nonseparable gradient approximation (NGA), as first employed in the N12 functional, we developed a gradient approximation for molecules (GAM) that is parameterized with a broader data set of molecular data than N12 and with smoothness constraints. By adding the kinetic energy density (τ) to the GAM functional, we developed a new meta-NGA called MN15-L that predicts accurate results for multi-reference systems especially for transition metal ligand binding energies. Adding 44% Hartree-Fock exchange to the MN15-L functional and optimizing the linear parameters of the functional in the presence of Hartree-Fock exchange, we obtained a non-local exchange-correlation functional called MN15 that predicts accurate results for a large variety of properties including single-reference systems, multi-reference systems, and noncovalent interactions. In this thesis we presents the following studies: (1) Introduction of Density Functional Theory (DFT), (2) Development of Minnesota Database 2015B, (3) The GAM Functional, (IV) The MN15-L Functional, (V) The MN15 Functional, and (VI) Applications of Kohn-Sham Density Functionals.
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    Exploring Structural Transitions in Perovskite Oxides, Orthovanadates and Square-net compounds using First-principles Calculations
    (2023-12) Saha, Amartyajyoti
    Understanding the crystal structure and symmetry is essential for gaining insights into the fundamental physical and chemical properties of a material, as well as its macroscopic behavior. Recent advancements in computational power and the development of advanced and reproducible density functional theory-based methods provide us with a powerful arsenal to explore crystal structures. This thesis involves a first-principles computational study of structural instabilities, phase transitions, and various electronic and thermal properties in four distinct families of materials. I use group-theoretical analysis to study the evolution of octahedral rotation and ferroelectric instabilities in Pnma perovskite oxides for multiple compounds under different strains. I explore the stability of the tetrahedral rotation and tilt in orthovanadates with palmierite structure, and the resulting phase transitions. In layered 112 compounds with 2D square nets, I study the instability of the square-net as well as its novel electronic properties. I also investigate the electronic and thermal properties of metallic delafossite, which show remarkable agreement with experiments. For each class of compounds, I use a combination of first-principles methods and group-theoretical analysis to explore the evolution of various instabilities and the resulting changes in structure and symmetry via lattice vibrations.
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    First-principles Study of Lattice Dynamics in Crystals
    (2022-06) Li, Shutong
    Lattice dynamics is a key component in solid state physics. It helps the understanding of many physical properties like structural phase transitions and ferroelectricity. Density functional theory, as a first-principles method, is used to investigate the lattice dynamics in this thesis. Followed by an introduction of density functional theory and lattice dynamics, I first study the strain-suppresed polarization switching barriers in layered perovskites. It is shown that the epitaxial strain is strongly coupled with the free energy of different crystal structures, which enables us to tune the energy difference between stable and transition states. The concept of distortion symmetry group is also utilized here to model the switching process accurately. Second, the idea of free-carriers-induced ferroelectricity is introduced. Free charge carriers is typically detrimental to proper ferroelectricity, but it is not the case for hybrid improper ferroelectrics. This unexpected phenomenon will be explained by the electron-enhancement of oxygen octahedral rotation. Group theory analysis and Landau free energy are also carefully looked into in this system. Third, the nature of chemical bonding in transition metal dichalcogenides (TMD) is investigated using Wannier functions. My DFPT results indicate anomalous ionic charges of HfS2 in the in-plane direction, which is also confirmed by infrared and Raman spectrum from our collaborators. The study of Wannier functions attributes this robust ionicity to the hybridization of Hf and S orbitals. Finally, this dissertation is concluded by a brief comment of future opportunities and challenges in this research field.
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    Modeling Chemical Reactions Mediated by Earth-Abundant Transition-Metal Complexes
    (2018-06) Dereli, Busra
    As many key elementary reactions having broad utility in chemistry have been found to be catalyzed at transition-metal centers, there has been enormous effort devoted to the design and optimization of such catalysts. In addition to empirical experiments that measure catalytic activity, theoretical chemistry offers a complementary approach to understanding mechanisms of catalysis and can be used to accelerate the design of second- and subsequent-generation catalysts with further improved properties. In the broader sense, catalysis science provides opportunities to explore and understand how catalysts work at the atomic scale by means of computation, synthesis, and characterization. The primary goal of this thesis to investigate the fundamental features of transition-metal-based systems and their roles in the activation mechanisms of hydrocarbon and biomass feedstocks via density functional theory (DFT) and wave function (WF) theory methods. Accordingly, this thesis presents (i) accurate prediction of gas-phase ionization energies of mononuclear copper complexes using high level quantum mechanical methods (Chapter 2), (ii) the effects of electronic perturbations on the copper-hydroxide complexes involved in C-H bond activation reactions (Chapter 3), (iii) catalytic decarbonylation of biomass-derived carboxylic acid derivatives to olefins (Chapter 4), (iv) the dual ligand role in selective decarbonylation of fatty-acid esters to linear α-olefins (Chapter 5).
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    Quantum chemical studies of actinides and lanthanides: from small molecules to nanoclusters
    (2013-06) Vlaisavljevich, Bess
    Research into actinides is of high interest because of their potential applications as an energy source and for the environmental implications therein. Global concern has arisen since the development of the actinide concept in the 1940s led to the industrial scale use of the commercial nuclear energy cycle and nuclear weapons production. Large quantities of waste have been generated from these processes inspiring efforts to address fundamental questions in actinide science. In this regard, the objective of this work is to use theory to provide insight and predictions into actinide chemistry, where experimental work is extremely challenging because of the intrinsic difficulties of the experiments themselves and the safety issues associated with this type of chemistry. This thesis is a collection of theoretical studies of actinide chemistry falling into three categories: quantum chemical and matrix isolation studies of small molecules, the electronic structure of organoactinide systems, and uranyl peroxide nanoclusters and other solid state actinide compounds. The work herein not only spans a wide range of systems size but also investigates a range of chemical problems. Various quantum chemical approaches have been employed. Wave function-based methods have been used to study the electronic structure of actinide containing molecules of small to middle-size. Among these methods, the complete active space self consistent field (CASSCF) approach with corrections from second-order perturbation theory (CASPT2), the generalized active space SCF (GASSCF) approach, and Møller-Plesset second-order perturbation theory (MP2) have been employed. Likewise, density functional theory (DFT) has been used along with analysis tools like bond energy decomposition, bond orders, and Bader's Atoms in Molecules. From these quantum chemical results, comparison with experimentally obtained structures and spectra are made.
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    Theoretical Insights into the Effects of Interfacial Electrolytes and Catalyst Characteristics on Reduction Chemistries
    (2022-05) Gorthy, Sahithi
    The limited reserves of fossil fuels, and rising concerns about global warming and climate change, have motivated the development of sustainable methods for catalytic systems. This thesis focuses on the electrocatalytic reduction of carbon dioxide (CO2) and the catalytic reduction of oxygen (O2) to value-added chemicals using environmentally-friendly processes.The electrochemical reduction of CO2 to energy-dense chemicals using renewable energy resources is attractive; however, lowering the associated overpotentials and improving selectivity at high current density outputs is imperative to become carbon-neutral. The work presented herein uses potential-dependent ab initio molecular dynamics and density functional theory methods to explore the role of the local reaction environment and the metal catalyst on CO2 reduction. Specifically, we examine the role of ionic liquids and alkaline electrolytes on CO2 activation and subsequent reduction. The calculations with ionic liquids reveal that their cations can stabilize negatively charged surface intermediates through hydrogen bonding, thereby lowering CO2 onset potential. Our simulations in potassium hydroxide solutions reveal that the hydroxide anion can adsorb on the cathode to promote electron transfer to the adsorbed CO2 radical, improving the reduction current density. The analysis of alkaline electrolytes with different anions indicates that the anion can play a dual role by promoting charge transfer and directly interacting with the adsorbed intermediates through hydrogen bonding or electrostatic interactions, thus changing reduction overpotential and current density. Further, we also study the formation of multi-carbon products on different copper facets under different operating conditions.Finally, we investigate the effect of bimetallic catalysts of gold and palladium on reducing oxygen to hydrogen peroxide selectively in aqueous environments. Theoretical calculations and experimental rate measurements indicate that solvent water molecules mediate oxygen reduction through proton-electron transfer steps and that the difference in the structural sensitivity for the formation of peroxide vs. water results in increased selectivity as palladium is isolated in gold. This thesis shows that both the solvent environment and the active catalyst play critical roles in determining the activity and selectivity of reduction reactions, and explicit solvent modeling is essential to accurately capture the interactions and understand the distinct roles played by each component.
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    Theoretical Insights into the Molecular Transformations Governing Redox Chemistry
    (2020-09) Udyavara, Sagar
    Redox reactions are one of the most prevalent chemistries and a ubiquitous component in naturally occurring processes as well as in the production of fuels and various chemicals involving pharmaceuticals, agrochemicals, and other industrially relevant chemicals. Research efforts in the past have focused on the development of safe and environmentally benign redox processes using alternative resources to deal with issues pertaining to global warming and depletion of the existing crude oil reserves. In addition, in the area of organic synthesis, new synthesis strategies are being developed to access important redox derived chemical intermediates via use of less toxic and easily available redox reagents. However, as is the case with most newly developed catalytic processes, the fundamental challenges associated with poor reaction selectivity and activity hinder the effectiveness of these new reactive transformations and technologies, making them less desirable compared to the currently used approaches. Understanding the molecular transformations occurring during these processes thus remains the key ingredient in the further development of these new processes. Gaining mechanistic and kinetic insights into the reaction would allow us to identify factors that control the selectivity and activity for the given reaction, which would then guide the development of more active and selective reaction systems. Along these lines, electronic structure calculations based mainly on density functional theory (DFT) has emerged as a powerful tool to model the reaction kinetics and also to gain important perspectives about the chemical reactions that are not observable experimentally. My dissertation thus focuses on using density functional theory (DFT) to gain important molecular level insights into the transformations that control the oxidation and reduction pathways occurring via thermal or electrocatalytic routes for five independent redox chemistries. Further, we also look into the salient features of redox chemistry that come into play during fabrication of electronic devices, particularly for MoS2 based transistor devices, which ultimately affect its ensuing performance. The first part of the thesis focuses on ab-initio study of three particular systems of interest in oxidation chemistry. Each of these studies separately report on three distinct catalytic features including the nature of the sites, molecularity of the catalyst, and the presence of surface coverages that affect the selectivity towards the desired product. The initial focus is on processes involving partial oxidation of feedstock chemicals which suffer from issues of low selectivity since the processes of over-oxidation is thermodynamically more favorable. One such example of a system that suffers from over-oxidation is sulfur oxidative coupling of methane (SOCM), which involves selective production of ethylene from methane using sulfur as an oxidant. Herein, we report a detailed kinetic and mechanistic study for SOCM reaction done over a sulfided Fe3O4 catalyst (FeS2). Experimental mechanistic analysis involving Delplot and contact time studies reveal a reaction network for SOCM that is different compared to the traditional OCM. Further computational analysis done for this reaction network suggests a site-specific formation of the selective C2 products and unselective carbon-di-sulfide, allowing for potential tuning of the reaction selectivity. The second system of interest that displays challenges with over-oxidation is the dehydrogenation of cyclohexanone. In this study, we examine the role of the homogeneity of the catalyst (Pd(DMSO)2(TFA)2) in selective control of the dehydrogenation reaction of cyclohexanone. Our DFT calculations indicate that the dehydrogenation proceeds via a rate determining intermolecular deprotonation step, activating the C-H bond at the alpha position. Using distortion-interaction analysis, the further activation of the cyclohexenone intermediate formed is shown to be inhibited due to its higher C-H bond strengths as well as due to its weaker interactions with the hydrogen abstracting entity, TFA- compared to cyclohexanone. Besides issues pertaining to over-oxidation affecting the selectivity, for reacting species with multiple reaction centres or functionalities, multiple parallel reaction pathways can emerge which could also lead to lowering of selectivity towards the desired intermediate. Primary electro-oxidation of glycerol over Pt catalyst is one such example wherein the product distribution is dependent on the carbon center that is being activated (primary versus secondary). Using DFT calculations, we hereby report the influence of coverages on directing the oxidation of the C-H bond at the primary versus the secondary position. We show that under high coverage conditions, oxidation at the primary C-H bond to form glyceraldehyde is preferred despite the stronger C-H bond strengths whereas under low coverage conditions, the weaker secondary C-H bond is preferentially activated to form dihydroxyacetone. We further examine the influence of the reaction conditions such as the pH, metal surface, and the operating potentials on the resultant product distribution for electrocatalytic glycerol oxidation over Pt. The second part of the thesis involves a mechanistic investigation of some important reduction chemistries, in particular - the hydrogen evolution reaction (HER) and the electrochemical Birch reduction. In the HER, we hereby examine MoS2 as a non-precious catalyst for use in fuel cells for production of hydrogen from water, and report enhancements in its electrocatalytic activity via application of an external electric field normal to the MoS2 electrode surface (back-gating). DFT studies aimed to understand the mechanistic nuances of the observed increased activity show that the excess electron densities induced via back-gating increase the binding energies of the hydrogen on the MoS2 surface, which in turn lead to improvements in the electrocatalytic activity of MoS2 for HER. In the next topic, we look at the Birch reduction process wherein we report a practical, safe, and scalable electrosynthetic strategy for the reduction of the arenes to dienes and for other similar reductive transformations and subsequently determine the mechanistic nuances of the reaction in the presence of various reagents and additives. Through electroanalytical and computational investigations, we have shown the reaction pathway to proceed via the reduction of the substrate near the electrode surface with the protonation step as the rate limiting step of the reaction. Further, we have deciphered the unique role of each of the reagents used in the study – electrolyte, lithium bromide (LiBr); solvent, tetrahydrofuran (THF); proton source, dimethyl urea (DMU); and the additive, tris(pyrrolidino)phosphoramide (TPPA), to understand how these components co-operatively aid in promoting the desired transformations towards higher reaction yields. In the final part of the thesis, diverging from the theme of redox reactions in chemical production, we investigate the influence of the redox reactions in device fabrication processes with the aim to mitigate some of the issues concerning the contact problem typically observed at the interface. We study here a system of MoS2-metal contacts, which have applications in development of sub-nanometric transistors. Via our “single-atom addition” approach used here, we show that the nature of the metal deposited strongly influences the resulting structural and electronic properties of the MoS2-metal interface. In conjunction with analytical scanning tunnelling electron microscopy (STEM) studies, we screen and characterize the interfacial properties for different metal contacts including Sc, Ti, Cu, In, and Au, with the aim to design systems that minimize the contact resistance at these interfaces.