Correlated description of multiple bonds using localized active orbitals

It is shown that the use of atomic-like active orbitals instead of symmetry-adapted ones is computationally interesting for the treatment of strongly correlated problems, when the zero-order description involves a large complete active space (CAS). Using localized active orbitals makes it possible to reduce the number of references to the leading valence bond configurations, and hence to achieve the calculation of the dynamical correlation energy for a lower cost. This benefit would be especially dramatic for remote active centers but it remains significant in the most difficult problem of strong multiple bonds, as illustrated on N2 and Mo2Cl84−. The valence bond analysis of the wavefunction makes clear the physical content of the non-dynamical correlation.