For a vanishing energy difference, ∆ i → 0, the amplitude T i now remains finite owing to the presence of ε in the denominator. This technique is very effective in removing intruder states from ground state calculations and it was proposed as a remedy to the ISP in CASPT2 by Roos and Andersson 23 . However, because ε uniformly shifts all the amplitudes of the firstorder wave function, it affects also the weight of states Φ i associated to large denominators, resulting in a second-order energy that strongly depends on it. In an attempt to decrease this dependence, a level-shift correction that tries to minimize the impact of the shift on large denominators was proposed. The expression presented in the original contribution 23