On the difference between variational and unitary coupled cluster theories.

  title={On the difference between variational and unitary coupled cluster theories.},
  author={Gaurav Harsha and Toru Shiozaki and Gustavo E. Scuseria},
  journal={The Journal of chemical physics},
  volume={148 4},
There have been assertions in the literature that the variational and unitary forms of coupled cluster theory lead to the same energy functional. Numerical evidence from previous authors was inconsistent with this claim, yet the small energy differences found between the two methods and the relatively large number of variational parameters precluded an unequivocal conclusion. Using the Lipkin Hamiltonian, we here present conclusive numerical evidence that the two theories yield different… 

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