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Updated: Oct 1, 2025
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Published on: April 8, 2020
Coupled cluster downfolding methods: The effect of double commutator terms on the accuracy of ground-state energies
Nicholas P Bauman1, Karol Kowalski1
1Physical Sciences and Computational Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
Abstract:
Downfolding coupled cluster techniques have recently been introduced into quantum chemistry as a tool for the dimensionality reduction of the many-body quantum problem. As opposed to earlier formulations in physics and chemistry based on the concept of effective Hamiltonians, the appearance of the downfolded Hamiltonians is a natural consequence of the single-reference exponential parameterization of the wave function. In this paper, we discuss the impact of higher-order terms originating in double commutators. In analogy to previous studies, we consider the case when only one- and two-body interactions are included in the downfolded Hamiltonians. We demonstrate the efficiency of the many-body expansions involving single and double commutators for the unitary extension of the downfolded Hamiltonians on the example of the beryllium atom, and bond-breaking processes in the Li2 and H2O molecules. For the H2O system, we also analyze energies obtained with downfolding procedures as functions of the active space size.
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