for an open-shell reference determinant in which the number of α and β electrons is different. In this case, α and β parts of the HF density are not the same; therefore, the mean-field Fock-like matrices from Eq. ( ) also do not have a proper symmetry of the different spin blocks. The h SOM F,2e z terms acquire additional unphysical contributions from the singlet part of the transition density matrix, which leads to artifacts illustrated numerically in Section III D.