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Electron correlation is the everlasting concern of people involved in ab initio Hartree-Fock calculations. If the investigation is restricted to the structure and properties of the ground state, the importance of correlation depends i) on the type of bonding in the molecule under scrutiny, and ii) on the level of accuracy requested for the calculation, that is, on the sensitivity of the investigated property. This sensitivity may be highly interrelated with the nature of the bonding: in the specific area of organometallic complexes, it has been recognized since more than a decade that no realistic description of multiple, direct metal-metal bonds can be obtained without an adequate treatment of the left-right correlation.1,2 Accidental near degeneracies connected for instance with the sd hybridization in complexes of Ni(0) should also be accounted for in a systematic way.3 Other cases where correlation is susceptible to qualitatively modify the description of the ground state can be detected from the occurrence of Hartree-Fock instability which makes the energy of the considered system symmetry-dependant.4,5 Introducing a systematic treatment of correlation for large systems in the frame of the Hartree-Fock methodology would lead to technical problems and to an intolerable computer cost. Our experience in the computation of the ground state electronic structure and properties of dimetallic complexes and of clusters with higher nuclearity argues for a flexible approach of the correlation problem.