Antiferromagnetic Exchange Interaction between Electrons on Degenerate   LUMOs in Benzene Dianion
  We discuss the ground state of Benzene dianion (Bz$^{2-}$) on the basis of the numerical diagonalization method of an effective model of $\pi$ orbitals. It is found that the ground state can be the spin singlet state, and the exchange coupling between LUMOs can be antiferromagnetic. 
Hiroyasu Matsuura, Kazumasa Miyake, and Hidetoshi Fukuyama
cond-mat.str-el cond-mat.mtrl-sci physics.atm-clus