In contrast to level shift techniques, its main purpose is to affect the computed result; specifically, to reduce the systematic underestimation of excitation energies by CASPT2 with the standard Fock operator. As this underestimation can be attributed to overestimation of the correlation energy for open-shell systems, it can be alleviated by correcting the denominator of the correlation energy (the second term in Eq. ( )) through modifying the zeroth-order Hamiltonian. The IPEA shift technique often also alleviates the intruder state problem, although this is not its primary objective.