UC Berkeley

Electronic excited states play two central roles in molecular chemistry: drivers of chemical change and reporters through spectroscopic measurements. While electronic ground states are theoretically well understood, excited states, and particularly excited states beyond the ground state linear response regime, require special treatment for both understanding of chemical phenomena and ab initio spectral predictions. In this work, we investigate tailoring state-specific electronic structure methodology to accurately predict X-ray absorption excitation energies both of closed shell and electronically excited molecules. X-ray absorption probes inner-shell core electrons and has become an powerful tool for studying chemical dynamics in time-resolved experiments. Many complications arise when describing the electronic structure of core-excited states, the most important of which is properly contracting the valence orbitals about the post-excitation core hole. We approach these challenges numerous ways. First, we utilize the latest advances in excited-state specific Quantum Monte Carlo (QMC) to calculate highly accurate, systematically improvable excitation energy predictions for small closed shell molecules, seeking to addresses whether QMC can serve in it's traditional role as a benchmark theoretical tool to test more approximate theories against. We then investigate how statistical uncertainty within QMC optimization can impact seemingly converged results when the wavefunction parameterization contains redundant parameters. Next, we address the accuracy of excited state mean field theory and it's perturbative extensions (ESMF and ESMP2) as a low cost alternative tool for predicting core excitation energies. Finally, we theoretically predict the time-resolved XAS spectra of the diradical photochemical ring opening of the furanone molecule. Through this work, we infer a novel spin-induced selection rule for core-to-unoccupied excitations of diradicals which should aid in the interpretation of time-resolved XAS experiments.