1. Contribution from The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-01, Japan, School of Informatics and Sciences, Nagoya University, Chikusa, Nagoya 464-01, Japan, Institut für Anorganische Chemie der Universität Würzburg Am Hubland, D-97074 Würzburg, Germany, and Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322