1. Departamento de Física de la Materia Condensada, Facultad de Física, Universidad de Santiago de Compostela, E-15782 Santiago de Compostela, Spain, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, Center for Computational Materials, Institute for Computational Engineering and Sciences, University of Texas, Austin, Texas 78712, and Departments of Physics and Chemical Engineering, University of Texas, Austin, Texas 78712