Relativistic Quantum Chemical Studies For Excited States And Thermodynamic Properties Of Uranium Compounds

Theoretical studies of actinide compounds are essential for elucidating their electronic structures and various physicochemical properties. There are still numerous open issues in this area and the difficulties are mainly due to significant relativistic effects and complicated electron correlation in these compounds. Despite of rich experimental data accumulated during the development of nuclear energy and environmental nuclear science, theoretical investigations on actinide compounds are far behind the experimental efforts. In order to explain experimental facts and be able to predict some of the properties with reasonable accuracy, it is mandatory to develop and calibrate accurate and efficient computational methodologies for application in actinide compounds. In this dissertation I have investigated a series of theoretical schemes applicable to actinide compounds, focusing on predictions of the key properties of uranium compounds relevant to nuclear industry, environmental science, and fundamental actinide chemistry.Investigations of the excited states of actinide compounds are one of challenges in actinide chemistry. An ab initio post-HF strategy for calculating excited states of small uranium compounds is proposed in this work. By using a relativistic effective core potential （RECP）, a combined MCSCF and RASSI/SO method is used to calculate the energies and other excitation parameters for low-lying excited states of UN₂, NUO+, and UF₆ molecules. Accurate potential energy surfaces （PES） for the excited states of UN₂ and NUO+ have been obtained. The electronic spectra for the 5 low-lying absorptions of UF₆ are simulated, which is in good agreement with the experimental observation. The largest error between the predicted and observed absorptions is no larger than 0.₂ eV, which is a significant improvement when comparing with the published data. Such a strategy is applicable for other actinide systems with comparable accuracy. TDDFT calculations with spin-orbit coupling （SOC） are also performed and large deviations in excited energies are found when comparing with the ab initio post-HF data as reference, indicating that DFT methods with proper handling of the self-interaction error （SIE） are needed for excited states of uranium compounds.To investigate whether DFT functionals can predict the thermochemical data of uranium oxides with open-shells, systematic ab initio and DFT calculations are carried out on UO, UO₂ and their univalent and divalent cations. 1₆ exchange-correlation （XC） functionals at different levels of the“Jacob Ladder”are selected for comparison. It is shown that with careful calibrated XC functionals, the average errors against the experimental data for structural parameters, bond-dissociation energies and ionized energies can be reduced to less than 5%. These benchmark researches provide important information on calculations of large uranium compounds with computationally less-expensive DFT methods.To investigate the accuracy of DFT methods in predicting the electron-transfer （ET） dynamics, selected DFT methods combined with Marcus ET theory have been applied to study the model system [（UO₂）₂（H₂O）₁₂]³⁺ with 4₂ atoms for the electron self-exchange process between uranyl ions in aqueous solution. The difference of the calculated energy barriers is less than 3 kJ·mol^-1 when comparing the DFT and CASPT₂ results. For such reaction the rate constant has been determined to lie between 0.089 to 0.31 l·mol^-1·s^-1, which is within the range of the experimental ones. The successful predictions of the ET parameters of selected uranium systems indicate that DFT methods with appropriate treatment of self-interaction correction （SIC） is promising for applications in large uranium systems.