1. Department of Physical and Theoretical Chemistry, Faculty of Natural Sciences, Comenius University, Mlynská Dolina, SK-84215 Bratislava, Slovakia, Fakultät für Physik and Center for Computational Materials Science, Universität Wien, Sensengasse 8/12, Wien 1090 Austria, and Équipe de Modélisation Quantique et Cristallographique, CRM2, Institut Jean Barriol, UMR 7036, CNRS - Nancy-Université, B.P. 239, F-54506 Vandœuvre-lès-Nancy, France