In the case of closed-shell references, the SOMF approximation of the two-electron part of SOC can be treated in the same way as the one-electron part. To illustrate the problem of open-shell references, let us first discuss the contributions from the h SOM F,2e z terms. In the case of a closed-shell reference, α and β parts of the HF density (or, in general, the density of the reference determinant) are the same, which makes h SOM F,2e z,pαqα equal to -h SOM F,2e z,pβqβ .