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arXiv · 2107.11260

Improving perturbation theory for open-shell molecules via self-consistency

Lan Nguyen Tran
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Abstract

We present an extension of our one-body M{\o}ller-Plesset second-order perturbation (OBMP2) method for open-shell systems. We derived the OBMP2 Hamiltonian through the canonical transformation followed by the cumulant approximation to reduce many-body operators into one-body ones. The resulting Hamiltonian consists of an uncorrelated Fock (unperturbed Hamiltonian) and a one-body correlation potential (perturbed Hamiltonian) composed of only double excitations. Molecular orbitals and associated energy levels are then relaxed via self-consistency, similar to Hartree-Fock, in the presence of the correlation at the MP2 level. We demonstrate the OBMP2 performance by considering two examples well known for requiring orbital optimization: bond breaking and isotropic hyperfine coupling constants. In contrast to non-iterative MP2, we show that OBMP2 can yield a smooth transition through the unrestriction point and accurately predict isotropic hyperfine coupling constants.

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Lan Nguyen Tran. 2021-07-23. Improving perturbation theory for open-shell molecules via self-consistency. https://arxiv.org/abs/2107.11260

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