MO CI calculations are carried out using an optimal space of valence virtual MOs obtained by means of a projection technique, as a linear combination of the AOs which are more occupied in the molecular Fock space. Localization of the occupied MOs and nonvalence virtual MOs is also achieved. The overall procedure is proven to be quite advantageous and well suited to obtain potential energy curves which keep the same physical meaning along the range of distances studied. Using a slightly better than double‐zeta quality basis set, a valence CAS‐CI, and selected CI wave function by the CIPSI algorithm have revealed a possible weak van der Waals interaction for the 2Σ+ state of CuCO, which remains when polarization functions are added to the basis set for the carbon and oxygen atoms. Even though the CuCO 2Π and CuCO+ 1Σ+ states are energetically close, the nature of the interactions is quite different, π bonding and mainly electrostatic, respectively. The results give further support to the view of the neutral metal–CO interaction as a balance of σ repulsion and π backbonding. However, it is proposed that the driving force for the positive ion metal–CO interaction becomes essentially electrostatic.