PRACTICAL REMARKS ON THE SELECTION OF THE ACTIVE SPACE IN THE CAS-SCFWAVE-FUNCTION

The effect of the separation of an N-electron molecular system into tw o subsystems, as in the CAS-SCF wavefunction, is presented and analyze d from both geometrical and electronic structure points of view. The M RD-CI natural orbital selected in an adequate way can be used to build a CAS-SCF wavefunction which incorporates only the strong electron co rrelation. We give as an example the electronic structure of carbonyl oxide.