Browsing by Subject "Electronic structure theory"

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    Active Space Methods In Electronic Structure theory and Applications To Gas Separations In Metal-Organic Frameworks
    (2019-06) Stoneburner, Samuel
    Active space methods such as complete active space self-consistent field theory (CASSCF) are applied to many systems of interest with a focus on the challenges in choosing orbitals for active spaces. The systematic exploration of active spaces is considered from the standpoint of theoretical development, specifically the benchmarking of generalized active space self-consistent field theory and SplitGAS on a variety of systems. Additionally, a “correlated participating orbital” active space selection scheme is applied to CASSCF and restricted active space self-consistent field theory followed by second-order perturbation theory (CASPT2 and RASPT2, respectively) for singlet-triplet splittings of diradical organic molecules. “πCPO” is introduced as an effective and economical option for π-system excitation energies. It is also demonstrated that multiconfiguration pair-density functional theory (MC-PDFT) can provide good agreement with CASPT2 at a much lower computational cost. The computational affordability of MC-PDFT is also shown through the calculation of the full spin ladder of Fe2S2 compounds for which second-order perturbation theory could only be performed for high-spin states. The effects of including high local exchange (HLE) modifications to the MC-PDFT exchange and correlation energies for the relative spin-state energies of several other iron complexes is examined. The remainder of the work features gas separations in metal-organic frameworks (MOFs), beginning with CO2 capture in a copper paddle-wheel MOF and continuing to metal-catecholates, which are studied using Kohn-Sham density functional theory and CASPT2 in comparisons of different first-row transition metals for the capture of toxic NO and for O2/N2 separation. Finally, a screening study identifies specific MOF structures for metal-catecholate modification as synthetic targets for the purpose of O2/N2 separation.
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    Electronic structure theory and multi-structural statistical thermodynamics for computational chemical kinetics.
    (2012-08) Papajak, Ewa
    This thesis involves the development and application of methods for accurate computational thermochemistry. It consists of two parts. The first part focuses on the accuracy of the electronic structure methods. In particular, various augmentation schemes for one-electron basis sets are presented and tested for density functional theory (DFT) calculations and for wave function theory (WFT) calculations. The relationship between diffuse basis functions and basis set superposition error is discussed. For WFT, we also compare the efficiency of conventional one-electron basis-sets to that of newly developed explicitly correlated methods. Various ways of approaching the complete basis set limit of WFT calculations are explained, and recommendations are made for the best ways of achieving balance between the basis set size, higher-order correlation, and relativistic corrections. Applications of this work include computation of barrier heights, reaction and bond energies, electron affinities, ionization potentials, and noncovalent interactions. The second part of this thesis focuses on the problem of incorporating multistructural effects and anharmonicity effects in the torsional modes into partition function calculations, especially by using a new multi-structural torsion (MS-T) method. Applications of the MS-T method include partition functions of molecules and radicals important for combustion research. These partition functions are used to obtain thermodynamic functions that are the most reliable results available to date for these molecules. The multi-structural approach is also applied to two kinetics problems: • the hydrogen abstraction from carbon-3 of 1-butanol by hydroperoxyl radical • the 1,5-hydrogen shift isomerization of the 1-butoxyl radical In both cases multi-structural effects play an important role in the final results.