After the application of some projections, Q γ,αβ⊥ θΘ has a dimension of N state × N CSF , where N CSF is the number of CSFs, so the computational cost is strongly dependent on the length of the CSF expansion for the wavefunction (i.e., the active space). Without IPEA shift, κ act,γ,αβ tu computed with the iterative approach reproduces the off-diagonal elements of the unrelaxed second-order correlated density matrix (Eq. ( )) in the active block, but for the active space, it may be computed by