1. Southern Regional Research Center, U. S. Department of Agriculture, 1100 Robert E. Lee Blvd., New Orleans, Louisiana 70124, Institut für Physikalische und Theoretische Chemie, Technische Universität Graz, Technikerstrasse 4, A-8010 Graz, Austria, and Department of Chemistry and Supercomputer Institute, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455-0431