Communication: Projected Hartree Fock theory as a polynomial similarity transformation theory of single excitations

@article{Qiu2016CommunicationPH,
  title={Communication: Projected Hartree Fock theory as a polynomial similarity transformation theory of single excitations},
  author={Yiheng Qiu and Thomas M. Henderson and Gustavo E. Scuseria},
  journal={Journal of Chemical Physics},
  year={2016},
  volume={145},
  pages={111102}
}
Spin-projected Hartree-Fock is written as a particle-hole excitation ansatz over a symmetry-adapted reference determinant. Remarkably, this expansion has an analytic expression that we were able to decipher. While the form of the polynomial expansion is universal, the excitation amplitudes need to be optimized. This is equivalent to the optimization of orbitals in the conventional projected Hartree-Fock framework of non-orthogonal determinants. Using the inverse of the particle-hole expansion… 
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