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This thesis examines the ground and excited electronic states of
the uranyl (UO22+) and uranate (UO42-) ions using Hartree-Fock
self-consistent field (HF SCF), multi-configuration self-consistent
field (MCSCF), and multi-reference single and double excitation
configuration interaction (MR-CISD) methods. The MR-CISD
calculation included spin-orbit operators. Molecular geometries
were obtained from self-consistent field (SCF), second-order
perturbation theory (MP2), and density functional theory (DFT)
geometry optimizations using the NWChem 4.01 massively parallel ab
initio software package. COLUMBUS version 5.8.1 was used to perform
in-depth analysis on the HF SCF, MCSCF, and MR-CISD potential
energy surfaces. Excited state calculations for the uranyl ion were
performed using both a large- and small core relativistic effective
core potential (RECP) in order to calibrate the method. This
calibration included comparison to previous theoretical and
experimental work on the uranyl ion. Uranate excited states were
performed using the small-core RECP as well as the methodology
developed using the uranyl ion.
An accurate and efficient hybrid Density Functional Theory (DFT)
and Multireference Configuration Interaction (MRCI) model for
computing electronic excitation energies in atoms and molecules was
developed. The utility of a hybrid method becomes apparent when
ground and excited states of large molecules, clusters of
molecules, or even moderately sized molecules containing heavy
element atoms are desired. In the case of large systems of lighter
elements, the hybrid method brings to bear the numerical efficiency
of the DFT method in computing the electron-electron dynamic
correlation, while including non-dynamical electronic correlation
via the Configuration Interaction (CI) calculation. Substantial
reductions in the size of the CI expansion necessary to obtain
accurate spectroscopic results are possible in the hybrid method.
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