The equilibrium geometry of F₂⁺ in its ground electronic state. A simple example of the effects of symmetry breaking on an observable molecular property

Abstract

A wide variety of theoretical methods have been applied to a very simple but notoriously difficult problem, the calculation of the bond distance in F₂ ⁺. All theoretical methods used the same basis set, the standard Huzinaga-Dunning double-zeta plus polarization (DZ + P) set, designated F (9s 5p 1d/4s 2p 1d). All methods which enforce inversion symmetry and go beyond second-order perturbation theory are qualitatively successful, giving bond distances within 005 Å of experiment. Methods not enforcing inversion symmetry are successful to within 003 Å if based on a restricted Hartree-Fock (RHF) starting point. When the wavefunction is not constrained to have inversion symmetry, methods based on an unrestricted Hartree-Fock (UHF) starting point are less satisfactory, yielding errors in the F₂ ⁺ bond distance ranging from 0-092 Å (full fourth-order perturbation theory, UMP4) to 0-850 Å (single-determinant UHF).