UC Berkeley

The development of predictive computational methods to simulate, and thus interpret the phenomena relevant to core spectroscopies is a vibrant and flourishing field. Until recently, computational core spectroscopy was under-developed relative to ground-state and even valence excited-state quantum chemistry. By the appeal of its broad range of modern experimental applications, this emerging sub-field of quantum chemistry is a fertile landscape for research. The work presented here grew in this landscape, and the different findings are bound together by the common theme of using of orbital-optimized references for the description of core excited states. After laying a foundation in Chapter 1, Chapter 2 explores schemes for the use of orbital-optimized references as ingredients for correlated calculations with the aim of providing high quality predictions of the energy required to induce a core excitation. The schemes proposed rely on single-reference coupled-cluster singles and doubles - long lauded for being the most affordable variant within the accurate coupled cluster formalism - and achieves a statistical performance on the order of 0.2 eV for core excitations in small organic molecules. Chapter 3 designs a model constructed within a generalized single-excitation CI framework, relying on orbital-optimized references, to describe the exotic core excited states present in ultra-fast pump-probe core spectroscopy experiments. Specifically, these are core excited states atop valence excited states used to discern the electronic dynamics during photochemical processes. As an efficient yet accurate zeroth order model enjoying explicit spin adaptation, we demonstrate the utility of our model by simulating an ultra-fast time-resolved core absorption spectra of acetylacetone that requires small shifts in energy to align with experiment, and is free from artifacts due to spin contamination. Chapter 5 uses the aforementioned model for an exciting photochemical application on Fe(CO)5, a textbook organometallic compound used to understand metal-ligand bonding in these type of complexes. With our valence-core excited state model, we help uncover for the first time the spectroscopic signatures of the metal-centered excited states of FeCO5 during photodissociation. Finally, Chapter 5 offers some concluding remarks and future outlook.