On the uniqueness of the constrained space orbital variation (CSOV) technique

Several CSOV analyses are performed for the 1Sigma(+) state of NiCO, and it is shown that the importance of the CO sigma donation, Ni pi back donation, and interunit polarizations are virtually independent of the order of the CSOV steps, provided that the open-shell 3d sigma and 4s Ni orbitals are orthogonalized to the CO. This order of orthogonalization is consistent with the polarization of the Ni observed in the unconstrained SCF wavefunction. If instead the CO is orthogonalized to the open-shell Ni orbitals, the frozen orbital repulsion and entire CSOV analysis becomes unphysical. A comparison of the SCF and CAS SCF descriptions for the NiCO 1Sigma(+) state shows the importance of the s to d promotion and sd hybridization in reducing the repulsion and increasing the Ni to CO pi bonding. For LiF, CSOV analyses starting from both the neutral and ionic asymptotes show the bonding to be predominantly Li(+) - F(-). These examples show the uniqueness of the CSOV decomposition.