A comparative study of Kohn–Sham, Brueckner and Hartree–Fock orbitals

Comparative studies of Kohn–Sham orbitals (KSOs), Brueckner orbitals (BOs) represented by the Brueckner-coupled cluster orbitals, and Hartree–Fock (HF) orbitals have been carried out by means of criteria defined in terms of indices characterizing distances between various equidimensional subspaces spanned by these orbitals. It was found that the KSO–BO distances are smaller than the KSO–HF ones. The former distances decrease when proceeding from the local, through the gradient-corrected, to the hybrid exchange-correlation potentials. However, even for the latter potentials there are systems for which the KSO–BO distances are larger than the BO–HF ones. Therefore, it seems to be difficult to uphold the recent arguments concerning the closeness of Brueckner orbitals and Kohn–Sham orbitals, including those of schemes with purely local potentials.