The Hartree-Fock dissociation of F₂

We examine the stability of the restricted Hartree-Fock (RHF) wave function for F₂ in the vicinity of the equilibrium internuclear distance (R=R_e) and the shape of the unrestricted Hartree-Fock (UHF) potential energy curve for the same system. The results depend on the basis set: With a split valence plus polarization basis, 6-31G(d), the RHF wave function is unstable at R_e, and the UHF potential curve is purely dissociative. When the basis is extended to 6-311+G(3d) or 6-311+G(3df), the RHF wave function becomes stable, and the UHF potential curve acquires a local maximum for R slightly (∼0.02 å) greater than R_e. The local maximum, however, is only 0.1 kcal/mol higher than the local minimum at R=R_e.