Reference 192
F. Bernardi, A. Bottoni, M. Olivucci, M. A. Robb, H. B. Schlegel and G. Tonachini, "Supra-Antara Paths Really Exist for 2+2 Cycloaddition Reactions? Analytical Computation of the MC-SCF Hessians for Transition States of C2H4 with C2H4, Singlet O2 and Ketene" J. Am. Chem. Soc. 110, 5993 (1988).