He 2+ 2 : A comparison between Hartree–Fock and density functional techniques

A comparison of Roothaan‐Hartree–Fock methods (both restricted and unrestricted) with density functional ones ( LCAO ‐ X α and cellular MS ‐ X αβ) is made using as test case the He 2 2 + molecular ion. It is shown the analogy that exists between RHF and symmetry‐adapted LCAO ‐ X α potential energy curves, as well as between UHF and symmetry‐unconstrained LCAO ‐ X α ones. The influence of symmetry adaptation on the overall behavior of the potential energy curve is also discussed. Finally, the difference in the behavior of the LCAO ‐ X α and cellular MS ‐ X αβ calculations is explained as an artifact of the space partitioning in the latter technique. It is concluded that LCAO ‐ X α method is superior to cellular MS ‐ X αβ because it requires less effort to reach the same results and that the general behavior is similar to UHF , although the former affords a better equilibrium bond distance and a worse energy barrier than the latter.