Practical remarks on the selection of the active space in the CAS-SCF wavefunction

The effect of the separation of an N-electron molecular system into two subsystems, as in the CAS-SC wavefunction, is presented and analyzed from both geometrical and electronic structure points of view. The MRD-CI natural orbital selected in an adequate way can be used to build a CAS-SCF wavefunction which incorporates only the strong electron correlation. We give as an example the electronic structure of carbonyl oxide.