Configuration interaction with Kohn–Sham orbitals and their relation to excited electronic states

Kohn–Sham (KS) orbitals in CH2, formaldehyde and acetone molecules were used as reference states for configuration interaction (CI) instead of the usual Hartree–Fock (HF) orbitals. A little difference in overall accuracy of electronic excitation energies was found between these schemes. However, analysis of the wave functions indicated that Slater determinant with the KS orbitals is more suitable for construction of the electronic states. Typically, the main expansion coefficients for the CI/KS procedure were closer to unity than those for HF. The difference was most pronounced for the lowest-energy transitions, while the two methods provided more comparable results for the higher-energy states. Similar behaviour of singlet and triplet states was observed. The results justify the common practice of using the KS determinant as a wave function, for example in sum-over-states theories.