1. Materials Theory and Computation Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-1349, Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, Klinik und Poliklinik für Nuklearmedizin, Universität GH Essen, Hufelandstrasse 55, D-45147 Essen, Germany, and Department of Chemistry, University of California, Davis, California 95616