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I know the reduced density matrix has the form: $$ \gamma(\alpha,\beta) = \langle\Psi_N|c^{\dagger}_{\alpha} c_{\beta}|\Psi_n \rangle $$ but when $\Psi_N$ is the configuration interaction singles (CIS) wavefunction, why are the occupied-virtual (OV) and virtual-occupied (VO) subblocks zero? I see multiple references state that it is due to Brillouins Theorem, but I don't entirely see that connection.

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