Projected Coupled Cluster Theory

Abstract

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 Hartree-Fock theory captures the essential physics of many kinds of strong correlations via symmetry breaking and restoration. In this work, we combine and try to retain the merits of these two methods by applying symmetry projection to broken symmetry coupled cluster wavefunctions. The non-orthogonal nature of states resulting from the application of symmetry projection operators furnishes particle-hole 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 Hartree-Fock.

Submitted to JCP. Extra figures appear in the ancillary file

References in corpus (7)

• Symmetry broken and restored coupled-cluster theory I. Rotational symmetry and angular momentum
• Merging symmetry projection methods with coupled cluster theory: Lessons from the Lipkin model Hamiltonian
• Projected Hartree Fock Theory as a Polynomial Similarity Transformation Theory of Single Excitations
• Projected Hartree-Fock as a Polynomial of Particle-Hole Excitations and Its Combination With Variational Coupled Cluster Theory
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• Combining symmetry collective states with coupled cluster theory: Lessons from the Agassi model Hamiltonian
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