Browsing by Subject "density functional theory"

Now showing 1 - 6 of 6
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    Ab Initio Modeling of the Structures and Catalytic Properties of Selected Mononuclear, Dinuclear, Supramolecular, and Periodic Metal-Organic Complexes
    (2016-07) League, Aaron
    Numerous varieties of metal-organic compounds have been synthesized to date, ranging in size from complexes containing only one or two metal atoms to supramolecular clusters, cages, and periodic latices. Metal-organic compounds can be tuned for a multitude of applications by modifying organic ligands and metal centers to select for desired size, shape, and electronic properties. Because of the complicated nature of many such systems, characterization of them can be a challenge. Mechanisms of reactivity, too, are notoriously difficult to characterize experimentally; as the saying goes, "If you can isolate the species, it probably isn't important to the mechanism." Theory can be of assistance in both cases. Herein, we investigate the properties of a variety of metal-organic systems, including mononuclear and dinuclear ruthenium water oxidation catalysts, supramolecular Cu(I) and Fe(II) host-guest complexes, and a metal-organic framework modified with single-site Ni(II) and Co(II). In each case, we are able to use \textit{ab initio} methods to provide some valuable insight into the system: for the water oxidation catalysts, we examine the effects on catalytic efficacy of modifications to the ligand systems in order to advise the development of even better catalysts; for the host-guest complexes, we help to explain the binding trends observed between small molecules that incorporate into the internal cavities of the cages; and for the metal-organic framework, we take steps toward elucidating the structure resulting from deposition of Ni(II), as well as giving insight into the mechanisms by which the deposited metals catalyze ethylene hydrogenation and oligomerization.
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    Developing a Model Chemistry for Multiconfiguration Pair-Density Functional Theory to Study Photochemistry and Molecular Interactions
    (2021-01) Bao, Jie
    Photochemical reaction, which starts by exciting a system into an electronically excited state, is ubiquitous, for example, in the atmosphere. This has made photochemical reactions a very interesting topic. Multiconfigurational pair-density functional theory (MC-PDFT) is a powerful and efficient method for studying photochemical processes. This method has proved very efficient compared with other wave function methods, such as multi-state complete active space second order perturbation theory (MS-CASPT2), especially for large systems. Successful as MC-PDFT is, there are some limitations that stop MC-PDFT from being applied to studying photochemistry problems. The first limitation is that, like other multireference methods, the performance of MC-PDFT depends on the quality of the reference wave function, which by convention is optimized by an active-space method, such as complete active space self-consistent field (CASSCF). The second limitation is that MC-PDFT is a single-state method that does not include state interaction between reference states. This means that MC-PDFT gives wrong topologies of potential energy surfaces, which are important in studying photochemical reactions. My work is focused on resolving these two limitations. We proposed the ABC scheme and the ABC2 scheme to automatically generate the active space that gives good-quality reference wave functions thus successfully reproducing vertical excitation energies obtained from experiments or high-level calculations. We proposed the extended multi-state PDFT (XMS-PDFT) and compressed-state multi-state PDFT (CMS-PDFT) as two options to introduce state-interaction in pair-density functional theory. Among two methods, XMS-PDFT is more efficient, while CMS-PDFT is more robust. Both methods proved successful in providing correct topologies of potential energy surfaces for a variety of systems.
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    Explorations of constructs for unconventional and topological superconductivities
    (2022-09) Heischmidt, Brett
    In recent years, topology has risen as a prominent topic of study within the physics community. At its core, topology is simply a classification system, where all objects within a particular class (or more formally, space) hold a common property. Physicists tend to find topology interesting for a few reasons. First, the classification system can be extremely neat (clean), as when an integral over a physical space comes out as an integer multiple of some constant. Second, interesting physical manifestations can arise when a system lives in one topological class compared to another. Third, other physical manifestations can arise when crossing between topological classes. This thesis work centers itself around various topologies. The central topology is that related to the phenomenon of Majorana Zero Modes (MZMs), which are superconducting excitations at the split between particles and holes (i.e., zero energy). The topological classes relevant here are arrangements of certain systems that give rise to the MZM. There is a secondary topology associated with MZMs tied to their use in so-called "topological quantum computing." In this type of quantum computing, excitations are moved around one another in such a way that they remember where they have been by accumulation of a particular phase. Due to the physical process and its inherent memory of its path, this process has been dubbed "braiding." Aligned with previous language, the topological classes here, then, are the braids. This work studies two systems within the above motivations, NbSe$_2$ and magnet-semiconductor interfaces. NbSe$_2$ is predicted to be a nodal topological superconductor, wherein classes within the topology are defined on the nodes in the Bogoliubov-de Gennes (BdG) spectrum. (By convention, no nodes is trivial, and presence of nodes gives "nontrivial" classes.) Further, MZMs are predicted to arise when the nodes are present. Another platform for realizing MZMs is a combination of a semiconducting nanowire, s-wave superconductor, and magnetic element. Realizing unambiguous signatures of MZMs has been particularly tricky, however, leading to substantial efforts to understand the interactions of the three elements. The magnet-semiconductor interface studies fit within this context. Chapter 1 introduces some concepts motivating this work. The first concept presented is topology in quantum mechanical systems followed by its tie to superconductivity. The next concepts that are presented are tied to unconventional superconductivity and are central to its use in quantum computing. Chapter 2 presents an experimental analysis of NbSe$_2$. After outlining some history and motivation, device and measurement specifics are described. A main result of two-fold anisotropy in magnetoresistant properties of the superconducting state is presented followed by multiple efforts to rule out trivial causes. With these ruled out, an interpretation is presented describing a competition of superconducting instabilities. Chapter 3 addresses quantum spin transport in InSb nanowires. InAs and InSb nanowires are introduced for their role in experimentally showing MZMs. Experimental work on VLS InSb is sketched, although the focus here is a brief description of simulations relevant to the experimental picture. Progress toward exploring other platforms for this work is then presented. Chapter 4 moves into a computational study of Heusler / III-V semiconductor interfaces, with the motivation of studying the semiconductor-magnet interface. Grounding concepts are presented, followed by computational details for two interfaces, Ti$_2$MnIn-InSb and Ni$_2$MnIn-InAs. Results are finally discussed. Chapter 5 summarizes the work.
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    High-Accuracy and Low-Cost Electronic Structure Theory for Strongly Correlated Systems
    (2023-12) Zhang, Dayou
    Electronic structure theory is a powerful tool to study chemical systems, but it is very challenging to apply accurately to strongly correlated systems. Despite significant recent progress, a high-accuracy and low-cost electronic structure theory for strongly correlated systems is not available. This is partly related to the scarcity of accurate reference data for developing and testing improved theories, but it is also due to insufficient fundamental understanding of the ingredients necessary for a theory of a strongly correlated system to be accurate. This thesis addresses these issues. It includes benchmark studies on the spin-splitting energy of transition metals and their use to test a variety of wave function theories and density functionals in Kohn–Sham density functional theory, providing guidance as to which electronic structure method might be accurate for practical calculations, as well as providing accurate reference data for future theory development. It also contains the development of several analysis tools for improving fundamental understanding of existing theories. The results provide insight into the sources of errors in correlation energies and into improvements of existing theories. Finally, based on the discoveries of this work, a new theoretical framework named multiconfiguration density-coherence functional theory (MC-DCFT) is presented. The new theoretical framework provides an alternative approach to combining multiconfiguration wave functions with density functional theory, making it a promising method for further development.
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    Magnetic Properties of Transition Metal Oxides from First-principles
    (2022-08) Gautreau, Dominique
    Due to the strong coupling between the spin, lattice, and orbital degrees of freedom, transition metal oxides exhibit a wealth of exotic phases, such as ferroelectricity, superconductivity, and magnetic ordering. In this thesis, I focus on the magnetic properties of three transition metal oxides. The first study I present in this thesis is on the botallackite cuprate Cu2(OH)3Br. I present the results for the excitation spectrum of the material, obtained through a combination of first-principles methods, linear spin wave theory and exact diagonalization. Our calculations of the dynamical structure factor highlight the coexistence of magnon and spinon excitations in the system, and our results qualitatively agree with experimental results obtained through inelastic neutron scattering.I then turn to the rare-earth titanate (RTiO3) compounds, which are well-known to transition from a predominantly ferromagnetic state to a predominantly G-type antiferromagnetic state with increasing rare earth radius. This extraordinary behavior arises from the high sensitivity of the exchange interactions to the crystal structure of RTiO3. As such, the rare-earth titanates are natural candidates for exploring the possibility of controlling a system’s magnetic behavior through the application of uniaxial or biaxial strain. I discuss the results of our comprehensive study of the rare-earth titanates, in which we used a combination of first-principles and analytical methods to show that the application of uniaxial or epitaxial strain in RTiO3 should lead to a host of magnetic and structural phase transitions. This study is then followed by a description of the collaborative works I have participated in, in which I provided first-principles and analytical calculations to complement experimental and theoretical analyses of RTiO3. I then discuss my contribution to the joint experimental and theoretical investigation of PYCCO. In this work, my coauthors demonstrate that simultaneous first-order spin-state/valence-state/metal-insulator transitions can be experimentally induced in PYCCO with applied epitaxial strain. Studying this system from first-principles, I provide evidence that the strain-tunable phase transitions in PYCCO are directly analogous to the first-order thermal phase transitions observed in PCCO.
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    A Study of Metal-Metal Bonds and Multiconfiguration Pair-Density Functional Theory
    (2017-07) Carlson, Rebecca
    The electronic structure and properties of various homo- and hetero-bimetallic complexes, which are relevant for small molecule activation, are discussed based on experimental and computational methods, including wave function theory and density functional theory. Due to their multiconfigurational nature, the theoretical challenges associated with transition metal complexes are also analyzed. A new method, Multiconfiguration Pair-Density Functional Theory (MC-PDFT), that is able to combine a multiconfigurational wave function with a density functional is introduced as a new way to treat these multiconfigurational systems and results on a wide variety of systems show, in general, good agreement with CASPT2. One of the fundamental quantities in MC-PDFT is the on-top pair density. An analytic solution of the on-top pair density is presented for H2 and as well as its relevance in understanding bond breaking.