This work has been supported by the NSF CHE-98-75824 and by a Research Corporation research innovations award # RI0218. The authors gratefully acknowledge a generous grant of computer time from the Hewlett-Packard Co. facility in Richardson, TX where many of the calculations have been performed on the site's high-performance computing facilities. The remainder of the calculations were performed on the HP V-Class system at Caltech under support of an NPACI allocation NYU 210.