Entanglement coupled cluster theory: Exact spin-adaptation

Sarai Dery Folkestad; Bendik Støa Sannes; Henrik Koch

doi:10.1063/5.0144694

Entanglement coupled cluster theory: Exact spin-adaptation

J Chem Phys. 2023 Jun 14;158(22):224114. doi: 10.1063/5.0144694.

Authors

Sarai Dery Folkestad¹, Bendik Støa Sannes¹, Henrik Koch^{1

2}

Affiliations

¹ Department of Chemistry, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
² Scuola Normale Superiore, Piazza dei Cavaleri 7, 56126 Pisa, Italy.

PMID: 37306953
DOI: 10.1063/5.0144694

Abstract

We present a novel framework for spin-adapted coupled cluster theory. The approach exploits the entanglement of an open-shell molecule with electrons in a non-interacting bath. Together, the molecule and the bath form a closed-shell system, and electron correlation can be included using the standard spin-adapted closed-shell coupled cluster formalism. A projection operator, which enforces conditions on the electrons in the bath, is used to obtain the desired state of the molecule. This entanglement coupled cluster theory is outlined, and proof-of-concept calculations for doublet states are reported. The approach is further extendable to open-shell systems with other values of the total spin.