Real-time coupled-cluster approaches for electronic multiphoton processes in atoms and molecules

This thesis explores time-dependent coupled cluster (TDCC) and time-dependent equation-of-motion coupled cluster (TD-EOM-CC) methods in quantum chemistry. As part of the thesis work, these methods were integrated into the eT quantum chemistry program for simulating atomic and molecular interactions with electromagnetic fields. The primary objective was to apply the methods to study processes observed at state-of-the-art laser facilities and evaluate their mathematical properties. A key aspect of the thesis involves simulations of molecular photon absorption from short UV and x-ray laser pulses in sequence, creating time-resolved depictions of electron behavior. This approach can be viewed as making movies of electron dynamics, capturing the intricacies of their behavior. Methods were also developed to reduce the computational cost of these simulations. Additionally, the thesis encompasses simulations of other multiphoton phenomena such as collective Rabi oscillations in multiple atoms and impulsive stimulated x-ray Raman scattering in strong fields. During the research, specific limitations were identified for each method in certain settings: diverging time-dependent parameters in TDCC and substantial discrepancies from expected results due to system size in TD-EOM-CC. However, in other settings, the methods behaved without these limitations. These contributions enhance our understanding of how to simulate fundamental light-driven chemical processes, which may aid our comprehension of phenomena like photosynthesis, radiation-induced DNA damage, and eyesight mechanisms.