1. Computational Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands, Philips Research Laboratories Aachen, Weisshausstrasse 2, D-52066 Aachen, Germany, and Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands