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MBE-CASSCF Approach for the Accurate Treatment of Large Active Spaces

26 Mar 2024arXiv:2403.17836links table onlyarchive 2025-07-28

Jonas Greiner, Ivan Gianni, Tommaso Nottoli, Filippo Lipparini, Janus J. Eriksen, Jürgen Gauss

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We present a novel implementation of the complete active space self-consistent field (CASSCF) method that makes use of the many-body expanded full configuration interaction (MBE-FCI) method to incrementally approximate electronic structures within large active spaces. On the basis of a hybrid first-order algorithm employing both Super-CI and quasi-Newton strategies for the optimization of molecular orbitals, we demonstrate both computational efficacy and high accuracy of the resulting MBE-CASSCF method. We assess the performance of our implementation on a set of established numerical tests before applying MBE-CASSCF in the investigation of the triplet-quintet spin gap of iron(II) porphyrin with active spaces as large as 50 electrons in 50 orbitals.

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