F4+:: A stable three-electron bonded complex and a challenge for standard ab initio computational methods

Qualitative considerations based on a simple molecular-orbital model point to the prediction that the radical cation complex F-4(+) should be stable, with a significant bonding energy, relative to dissociation to the Fz and F-2(+) fragments. A rectangular minimum is indeed characterized at the ab initio MP2 level, as well as a linear conformer displaying nearly free rotation about the central F-F bond. The Moller-Plesset results are, however, shown to be inconsistent and very poorly converged as the order of perturbation is increased, and to be subject to very strong spin contamination with additional problems of local symmetry breaking in the linear conformer. Higher level calculations of the quadratic configuration interaction or coupled-cluster type, with inclusion of single and double excitations and perturbative treatment of triples (QCISD(T) and CCSD(T)), are performed in their spin-unrestricted and spin-restricted forms. All high-level calculations yield optimized geometries and bonding energies which are consistent with each other and point to the firm conclusion that the F-4(+) complex should be stable either in a rectangular or in a linear conformation, with dissociation energies of the order of 13-16 kcal/mol.