Projected coupled cluster theory
Coupled cluster theory is the method of choice for weakly correlated systems. But in the strongly correlated regime, it faces a symmetry dilemma, where it either completely fails to describe the system or has to artificially break certain symmetries. On the other hand, projected HartreeFock theory captures the essential physics of many kinds of strong correlations via symmetry breaking and restoration. Here, we combine and try to retain the merits of these two methods by applying symmetry projection to broken symmetry coupled cluster wave functions. The nonorthogonal nature of states resulting from the application of symmetry projection operators furnishes particlehole excitations to all orders, thus creating an obstacle for the exact evaluation of overlaps. Here we provide a solution via a disentanglement framework theory that can be approximated rigorously and systematically. Results of projected coupled cluster theory are presented for molecules and the Hubbard model, showing that spin projection significantly improves unrestricted coupled cluster theory while restoring good quantum numbers. The energy of projected coupled cluster theory reduces to the unprojected one in the thermodynamic limit, albeit at a much slower rate than projected HartreeFock.
 Authors:

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 Rice Univ., Houston, TX (United States). Dept. of Chemistry
 Rice Univ., Houston, TX (United States). Dept. of Chemistry and Dept. of Physics and Astronomy
 Publication Date:
 Grant/Contract Number:
 SC0012575; CHE1462434
 Type:
 Accepted Manuscript
 Journal Name:
 Journal of Chemical Physics
 Additional Journal Information:
 Journal Volume: 147; Journal Issue: 6; Journal ID: ISSN 00219606
 Publisher:
 American Institute of Physics (AIP)
 Research Org:
 Temple Univ., Philadelphia, PA (United States). Center for the Computational Design of Functional Layered Materials (CCDM); Rice Univ., Houston, TX (United States)
 Sponsoring Org:
 USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC22); National Science Foundation (NSF); Welch Foundation
 Country of Publication:
 United States
 Language:
 English
 Subject:
 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; operator theory; Hubbard parameter; full configuration interaction; coupledcluster methods; many electron systems; zero point energy; correlationconsistent basis sets; self consistent field methods; correlated electrons; statistical thermodynamics
 OSTI Identifier:
 1474042
 Alternate Identifier(s):
 OSTI ID: 1374781
Qiu, Yiheng, Henderson, Thomas M., Zhao, Jinmo, and Scuseria, Gustavo E.. Projected coupled cluster theory. United States: N. p.,
Web. doi:10.1063/1.4991020.
Qiu, Yiheng, Henderson, Thomas M., Zhao, Jinmo, & Scuseria, Gustavo E.. Projected coupled cluster theory. United States. doi:10.1063/1.4991020.
Qiu, Yiheng, Henderson, Thomas M., Zhao, Jinmo, and Scuseria, Gustavo E.. 2017.
"Projected coupled cluster theory". United States.
doi:10.1063/1.4991020. https://www.osti.gov/servlets/purl/1474042.
@article{osti_1474042,
title = {Projected coupled cluster theory},
author = {Qiu, Yiheng and Henderson, Thomas M. and Zhao, Jinmo and Scuseria, Gustavo E.},
abstractNote = {Coupled cluster theory is the method of choice for weakly correlated systems. But in the strongly correlated regime, it faces a symmetry dilemma, where it either completely fails to describe the system or has to artificially break certain symmetries. On the other hand, projected HartreeFock theory captures the essential physics of many kinds of strong correlations via symmetry breaking and restoration. Here, we combine and try to retain the merits of these two methods by applying symmetry projection to broken symmetry coupled cluster wave functions. The nonorthogonal nature of states resulting from the application of symmetry projection operators furnishes particlehole excitations to all orders, thus creating an obstacle for the exact evaluation of overlaps. Here we provide a solution via a disentanglement framework theory that can be approximated rigorously and systematically. Results of projected coupled cluster theory are presented for molecules and the Hubbard model, showing that spin projection significantly improves unrestricted coupled cluster theory while restoring good quantum numbers. The energy of projected coupled cluster theory reduces to the unprojected one in the thermodynamic limit, albeit at a much slower rate than projected HartreeFock.},
doi = {10.1063/1.4991020},
journal = {Journal of Chemical Physics},
number = 6,
volume = 147,
place = {United States},
year = {2017},
month = {8}
}