Note that these contributions can analogously be constructed by contracting over the ovov block of the ERI instead. Thus, their interpretation is twofold: Choosing a basis in which Ĥ(0) is diagonal, we can consider the process of ionisation from an orbital i as well as electron attachment into an orbital a. The A 3 term corresponds to the hole correlation energy of orbital i, while the B 3 term yields the particle correlation energy of orbital a. For a more detailed analysis of this interpretation, we refer the reader to Ref. 47. Building these terms from a contraction of the amplitudes over the ovov block of the ERI, we can determine that they also encode the particlehole Coulomb interaction between the doubly excited configuration and the unperturbed ground state. These contributions have previously been identified as central missing pieces in the energy denominator of MP2, , suggesting that optimal values for these parameters may be on the order of 1.