Qiu, YihengHenderson, Thomas M.Scuseria, Gustavo E.2017-06-052017-06-052016Qiu, Yiheng, Henderson, Thomas M. and Scuseria, Gustavo E.. "Communication: Projected Hartree Fock theory as a polynomial similarity transformation theory of single excitations." <i>The Journal of Chemical Physics,</i> 145, no. 11 (2016) AIP Publishing LLC.: http://dx.doi.org/10.1063/1.4963082.https://hdl.handle.net/1911/94781Spin-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, we similarity transform the Hamiltonian in a coupled-cluster style theory. The left eigenvector of the non-Hermitian Hamiltonian is constructed in a similar particle-hole expansion fashion, and we show that to numerically reproduce variational projected Hartree-Fock results, one needs as many pair excitations in the bra as the number of strongly correlated entangled pairs in the system. This single-excitation polynomial similarity transformation theory is an alternative to our recently presented double excitation theory, but supports projected Hartree-Fock and coupled cluster simultaneously rather than interpolating between them.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlehttp://dx.doi.org/10.1063/1.4963082