Symmetry breaking in spin-restricted Hartree-Fock solutions: the case of the C2 molecule and the N2+ and F2+ cations

We examine a spin-preserving stability of restricted Hartree-Fock (RHF) and open-shell RHF (ROHF) solutions for homonuclear diatomic species, namely the molecule C-2 and the N-2(+) and F-2(+) cations, in the entire relevant range of internuclear separations. In the presence of respective singlet, doublet, or triplet instabilities we explore the implied broken-symmetry (BS) solutions and check their stability. We also address the occurrence of vanishing roots rendered by the stability problem in the case of BS solutions. Since for homonuclear diatomic species the space symmetry of the nuclear framework cannot be broken, the only possible space symmetry breaking involves either the electron charge distribution or the rotational symmetry about the internuclear axis. Such BS solutions arise primarily due to the symmetry breaking of the valence, mostly frontier, molecular orbitals, which approach atomic-type orbitals in the dissociation limit. The resulting BS RHF or ROHF solutions yield much more realistic potential energy curves (PECs) than do the symmetry-adapted (SA) solutions. For the sake of comparison, we also generated corresponding PECs using the density functional theory (DFT). Finally, we examine the role of BS molecular orbitals in post-HF correlated approaches to the many-electron problem, specifically in the computation of equilibrium properties using the coupled-cluster method with singles and doubles (CCSD) and its perturbatively corrected version for triples, the CCSD(T) method.