2025 Theses Doctoral

Correlated electronic structure theory for interfacial chemistry and excited states of extended systems

Vo, Ethan Anh

I begin by discussing the fundamentals of wave function theory, focusing on Hartree–Fock as the foundation of most quantum chemistry methods. I then provide details on basis sets and the formalism of second quantization, followed by an overview of configuration interaction. Building on this, I present the formalism of coupled-cluster theory and equation-of-motion coupled-cluster theory, and finally, I describe how these methods can be extended to treat periodic systems.

In Chapter 2, I explore valence excitations of semiconductors and insulators with correlated wave function theory. I calculate the band gaps of 12 inorganic semiconductors and insulators composed of first- through third-row elements using periodic equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) and atom-centered triple-zeta basis sets with up to 64 k-points. I analyze convergence with respect to orbital and k-point sampling, applying composite corrections and extrapolations to obtain final values. At the end of this chapter, I discover the performance of EOM-CCSD relative to workhorse methods in the community and how it fares against approximate excited state wave function methods.

Chapter 3, I report core binding energies for K-edge and L-edge transitions in simple semiconducting and insulating solids using periodic EOM-CCSD. My all-electron calculations employ triple-zeta basis sets with core correlation and Brillouin zone sampling of up to 4 x 4 x 4 k-points. Final values are obtained through composite corrections and extrapolation to the thermodynamic limit, yielding errors comparable to the accuracy of CCSD for molecular systems. The low-scaling approximation to EOM-CCSD achieves slightly reduced accuracy, but at significantly lower computational cost.

In the final chapter, I apply density functional theory and coupled cluster theory to investigate electrolyte decomposition on lithium metal surfaces, a key phenomenon in energy materials science. To enable the use of mature molecular quantum chemistry methods, I segment the adsorbed molecule–lithium system into molecular clusters. I find that even small, computationally tractable clusters, when combined with composite corrections from basis set and method refinements, can serve as an effective tool for identifying high-performing functionals and for parameterizing machine-learned force fields.