X-ray absorption spectra (XAS) is a method used to investigate atomic local structure and electronic states. Coupled cluster method is a numerical method used for describing many-body systems and electron correlation in a wavefunction. When equation-of-motion coupled cluster is used in XAS calculations, the ground state is applied to the excitation operator, which excites or ionizes the electron. This causes a large orbital relaxation error, normally ~5 eV, which leads to the need for triple excitations in order to obtain accurate results.
This dissertation introduces a coupled cluster method that uses "transition potential" reference orbitals to reduce the orbital relaxation error and help with error cancellation, called transition-potential coupled cluster (TP-CC), that is tested on our 14 small molecule data set. Then, the TP-CC fractional core orbital occupation number is optimized for a specific element and tested on the data set. The results of the optimized core orbital occupation number are utilized in nucleobase x-ray absorption K-edge spectra calculations and compared with experimental data. Another coupled cluster method is introduced to address the issue of orbital relaxation through the addition of triples excitation only in the core ionization potential. The similarity-transformed equation-of-motion coupled cluster (STEOM-CC) method is used, with the additional inclusion of core-valence separation (CVS) and correlation of triple excitations only within the calculation of core ionization energies, called CVS-STEOM-CCSD+cT. Our new method, CVS-STEOM-CCSD+cT is tested on our data set and compared to previously developed methods. Then, transition moments for CVS-STEOM-CCSD+cT excited states are implemented and tested on our data set. Lastly, tensor hypercontraction for open-shell systems is implemented and tested on molecules such as radicals, bond cleavages, and solvation shells, and the errors are compared to the errors obtained when using closed-shell tensor hypercontraction.
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Simons, Megan, "New Methods for Core-Hole Spectroscopy Based on Coupled Cluster Theory" (2023). Chemistry Theses and Dissertations. 36.