Approaching Accurate Description of Molecular Spectroscopies with Multi-reference Electronic Structure Methods

Abstract

In this dissertation we aim to approach an accurate description of molecular spectroscopy with multi-reference electronic structure methods, for the investigation of real-world problems. Among the challenges that our society is facing now, heavy element chemistry and radiation problems are gaining more attention due to their importance in scientific discovery and technology innovations, requiring a fundamental understanding from the perspective of theoretical chemistry. We start with an introduction of current quantum chemistry approaches, mainly centered on the computation of electronic transitions giving rise to spectroscopic signatures. The first part of this dissertation focuses on the development on relativistic post-HF electronic structure methods for heavy element spectroscopy. A brief review of relativistic Hamiltonians is given to address the significant work in the field of relativistic electronic structure methods, as well as their linkage and connections, followed by a description of the formalism for the prediction of spectroscopic intensities in the framework of exact-two- component configuration interaction and equation of motion coupled cluster theories. A determinant based Kramers-unrestricted exact-two-component multi-reference second order perturbation theory, which variationally includes relativistic corrections with a perturbative account of dynamic correlation is developed and benchmarked, offering an accurate yet computationally efficient alternative to capture the special relativity and multi-reference nature. The more rigorous four-component multi-reference electronic structure methods including Breit interaction and their benchmark results are presented, representing the most accurate many-body theories before going into the genuine relativistic quantum-electrodynamics theory. In the second part, we switch gears and focus on the radiation problem — water radiolysis — to explore the origin of reactive species produced by ionizing radiation in aqueous systems. A more complete picture of the ultrafast dynamics and reactive events initiated by photoionization of pure water is provided in an ab initio Ehrenfest dynamic study. Stepping beyond outer-valence ionization, the irradiated processes upon ionization of the entire valence band of water is targeted with the first attosecond X-ray pump/X-ray probe transient absorption study in condensed phase, with multi-reference configuration interaction method unraveling the spectroscopic signatures.