Electron correlation treatment via many-body expansions
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Abstract
This thesis has significantly advanced the many-body expanded full configuration interaction (MBE-FCI) method, improving its efficiency and accuracy in computing electronic correlation energies across diverse molecular systems. The enhancements prioritize accessibility, versatility, and performance of the method, enabling massively parallel computations exceeding chemical accuracy. Additionally, the many-body expanded complete active space self-consistent field method was developed, providing a framework for addressing large active spaces in statically correlated systems. This approach was applied to investigate the triplet-quintet spin gap of the iron(II) porphyrin complex, using systematically expanded active spaces of up to 50 electrons in 50 orbitals. To optimize MBE-FCI, orbital clustering and screening algorithms were introduced, reducing computational demands while preserving accuracy and ensuring robust error control. An automated algorithm for identifying reference active spaces enables the treatment of both statically and dynamically correlated systems. Furthermore, a novel symmetrization algorithm for localized molecular orbitals facilitates efficient symmetry exploitation in arbitrary molecular point groups. These advancements enable highly accurate ground-state energy calculations for benzene in a cc-pVTZ basis set, setting an unprecedented standard of precision for systems of this size. Collectively, this work establishes MBE-based methods as transformative tools for high-accuracy quantum chemical studies.