Electron correlation and magnetism: A simple molecular orbital approach for predicting ground-state spins of conjugated hydrocarbons

A simple molecular orbital (MO) method, the unrestricted Hartree-Fock (UHF) Hubbard method, is proposed to predict the spin preferences of conjugated hydrocarbons. It is shown in this work that this method can account well for ground-state spin multiplicities of not only neutral alternant and nonalternant systems but also ion species. More importantly, the underlying relation between the electron correlation and magnetism in various conjugated systems with different topological characters is revealed by using the present UHF approach. For alternant systems, the magnitude of electron correlation is found to be less important in determining their ground-state spins. Whereas for some nonalternant and ion systems, their ground-state spin multiplicities are shown to depend strongly on the magnitude of electron correlation. In addition, the applicability and limitations of some currently used semiempirical models are analyzed in a unified way.