I wanted to ask if it is true that a valid parametrization of a generic (unnormalized) state in the Fock space of spin-$1/2$ fermions can be written as: $$|\psi>= \prod_{j=1}^L[\alpha_j +\beta_j \hat c^{+}_{j,-}+\gamma_j \hat c^{+}_{j,+}+\delta_j \hat c^+_{j,-}\hat c^+_{j,+}]|0>,$$ where I have introduced the fermionic creation operators at site $j$ with spin $\sigma=\pm$, and the terms multiplying them are arbitrary coefficients. I believe it is not, since it captures only product states. Thanks in advance for the help!
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$\begingroup$ @Mauricio I think it is needed in order to include the states with double occupancy on the same site (with opposite spins). $\endgroup$– lgottaCommented Oct 19, 2023 at 12:43
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$\begingroup$ @Mauricio the OP is correct. A spin half orbital can be unfilled, spin up, spin down, or filled both spin up and down, and then no more. $\endgroup$– naturallyInconsistentCommented Oct 19, 2023 at 16:48
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