erefore, the renormalization factor ful lls the condition 0 ≤ Z ≤ 1, and its actual computation does not involve any additional cost when computed alongside E as they involve the same quantities. is renormalization procedure of the second-order correction, that we have named rPT2, bears obvious similarities with the computation of quasiparticle energies within the G 0 W 0 method. Practically, the e ect of rPT2 is to damp the value of E for small wave functions. Indeed, when N det is small, the sum E (0) + E usually overestimates (in magnitude) the FCI energy, yielding a pronounced non-linear behavior of the sCI+PT2 energy. Consequently, by computing instead the (renormalized) energy E (0) + Z E , one observes a much more linear behavior of the energy, hence an easier extrapolation to the FCI limit. Its practical usefulness is illustrated in Sec. VI B.