The Valence Isomerization of Cyclooctatetraene to Semibullvalene We are most grateful to "Servicios Informáticos de la Universidad de Alcalá", to Silicon Graphics, Inc. (Boston, MA) for computer facilities, and to Professors José-Luis Abboud (Instituto de Química Física "Rocasolano", C.S.I.C) and Julio Álvarez-Builla (Departamento de Química Organica, Universidad de Alcalá) for their generous and continuous support of our work. O.C. acknowledges financial support by the University of Alcalá for a short stay in the King's College, London, and the D.G.E.S. (project number PM97-0074).