Coupled Cluster Downfolding Methods: the effect of double commutator terms on the accuracy of ground-state energies

• URL: http://arxiv.org/abs/2110.12077v1
• Date: Fri, 22 Oct 2021 21:49:45 GMT
• Title: Coupled Cluster Downfolding Methods: the effect of double commutator terms on the accuracy of ground-state energies
• Authors: Nicholas P. Bauman, Karol Kowalski
• Abstract summary: We discuss the impact of higher-order terms originating in double commutators on the appearance of 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.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Downfolding coupled cluster (CC) 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 parametrization 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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