Understanding electronic excitation, photoelectron, and multiphoton ionization spectra, particularly those involving dark and strongly correlated states, or transition metals, as well as catalytic, structural, and electronic properties of gold nanoparticles pose significant challenges for theory and experiment. The existing experimental techniques may not be sufficiently powerful to provide definitive information on their own, whereas an accurate treatment of the relevant many-electron correlation effects required in theoretical analyses may be far from obvious. In this dissertation, we describe several high-level ab initio computational studies employing the completely renormalized (CR) and active-space coupled-cluster (CC) and equation-of-motion CC (EOMCC) approaches and the extensions of the EOMCC theory to open-shell systems around closed shells defining the electron-attached (EA) and ionized (IP) EOMCC frameworks, which demonstrate the transformative role these novel electronic structure methods developed in our group have played in understanding the previously unexplained experiments and phenomena. They include (i) challenging electronic spectra of the CNC, C2N, N3, and NCO molecules and the photoelectron spectrum of Au3− nanoparticle examined with the EA- and IP-EOMCC approaches, especially those invented in our group, (ii) the discovery of the doubly excited state of azulene below the ionization threshold, which mediates the 1 + 2’ multiphoton ionization experiments resulting in clear Rydberg fingerprint spectra, where the CR-EOMCC formalism developed in our group played a crucial role, (iii) the detailed investigation of the mechanism and energetics of the aerobic oxidation of methanol on Au8− particle, which benefited from the application of the ground-state CR-CC methodology, developed by our group as well, and (iv) definitive CR-CC and active-space CC studies showing that the ground state of 1,2,3,4-cyclobutanetetraone, which is characterized by densely spaced low-lying states, is a triplet, in agreement with the recently recorded photodetachment spectrum. These cutting-edge computational studies are accompanied by advances in CC/EOMCC algorithms and methodologies, including the development of parallel numerical energy gradients and second derivatives for fast geometry optimizations and harmonic vibrational frequency calculations at any CC/EOMCC level, allowing us to establish the geometries and relative energies of the low-energy isomers of the controversial Au8 particle, and the implementation of the unrestricted Hartree-Fock-based (UHF-based) CR-CC(2,3) approach, allowing us to show that unlike the popular CCSD(T) approach, which is very sensitive to the type of the reference determinant employed in the calculations, failing in bond-breaking situations when the restricted Hartree-Fock (RHF) reference is used and displaying poor behavior at intermediate nuclear separations with UHF references, its CR-CC(2,3) counterpart provides a robust description regardless of the reference type (RHF or UHF), with the spin-adapted RHF-based CR-CC(2,3) results being most accurate in the examined bond dissociation cases.
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