1. Contribution from the Departament de Química Física, Facultat de Química i Centre Especial de Recerca en Química Teórica, Parc Científic, Universitat de Barcelona, Av. Diagonal 647, Barcelona 08028, Spain, Department of Physics, University of Texas at El Paso, El Paso, Texas, 79968, and Center for Computational Materials Science, Code 6392, Naval Research Laboratory, Washington, D.C. 20375-5000