Page 61 - Strategies and Applications in Quantum Chemistry From Molecular Astrophysics to Molecular Engineer

46 O. PARISEL AND Y. ELLINGER

We emphasize that the Rydberg states are included in the variational MCSCF treatment in
order not to be undercorrelated relative to the valence states [48,49]. Furthermore, it is seen
that all the previous distributions are coupled to diexcitations from the to the
orbital: that way, we account for the non-dynamical repolarization of the polar bond in
a GVB-like approach.
Finally, we also account for an explicit relaxation of both lone pairs of by the
inclusion of corresponding correlating orbitals [50] and treat them at a GVB-like level.
The final space spanned by these distributions can then roughly be seen as a product of
CAS, GVB and MCSCF spaces. It accounts for all effects supposed to be essential to get a
good zeroth-order description of the ground and the excited valence and Rydberg
states.The dynamical correlation of the 8 electrons included in this variational treatment
will be recovered, in the ground state and in the excited states, with the perturbation, and so
will be the correlation energy arising from the core and from the remaining electrons.
The perturbation will also account for any coupling not explicitely included in the
distributions presented in Table 1.