Papajak, Ewa2012-11-142012-11-142012-08https://hdl.handle.net/11299/139021University of Minnesota Ph.D. dissertation. August 2012. Major: Chemical Physics. Advisor: Donald G. Truhlar. 1 computer file (PDF); xiii, 366 pages.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.en-USChemical physicsElectronic structure theoryStatistical mechanicsTheoretical chemistryThermochemistryThesis or Dissertation