However, the CASPT2-IPEA energy is not invariant with respect to rotations among active MOs (Eq. ( )), so we need to obtain a term that is capable of making the Lagrangian stationary with respect to rotations among active MOs and also determine κ act,γ,αβ tu to satisfy the above equation. These orbital rotations are redundant at the SCF level, and therefore the rotation cannot be uniquely determined by solving the standard Z-vector equation introduced later (Eq. ( )). Moreover, in contrast to the inactive and secondary orbitals, the rotation parameter (Lagrange multiplier) cannot be determined with (65) because of the linear dependence between orbital and state rotation parameters. On the other hand, κ core,γ,αβ ij can be determined with