1. Contribution from the Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602, Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium, and Institute of New Energy Material Chemistry, Institute of Scientific Computing, Nankai University, Tianjin 300071, P. R. China