On the Ordering of Orbital Energies in High-Spin ROHF

The restricted open-shell Hartree-Fock (ROHF) method is a standard tool used by quantum chemists for studying molecules with unpaired electrons In this work a problem with some implementations of the ROHF. method is presented along with an elegant solution. The ground state (2)A(2) potential energy surface of the 5,5'-(4H, 4H')-spirobi[cyclopenta[c]pyrrole]-2,2'6,6'-tetrahydro cation is the molecular test case, which elucidates the underlying problem For this molecule, four distinct ROHF perturbation theories yield smooth (and parallel) potential energy curves The arbitrariness of the ROHF orbital energies is illustrated with diatomic CN. The method proposed will also fix Aufbau principle violations reported by Plakhutin and Davidson [Plakhutin, B N, Davidson, E R J. Phys. Chem. A 2009, 113, 12386-12395]