where either E real shift and E imaginary shift is provided as a parameter, and Ĥ(0) D is the diagonal part of the zeroth-order Hamiltonian. As detailed below, this shift term has nothing to do with the IPEA shift. The unshifted second-order perturbation energy E PT2 γ , with which the SS-CASPT2 energy is computed, is obtained by removing the shift correction: