1. Faculty of Physics, Department of Astronomy-Astrophysics-Mechanics, University of Athens 1 , Panepistemiopolis, Athens 157 83, Greece
2. NITheCS, National Institute for Theoretical and Computational Sciences 2 , Pietermaritzburg, KwaZulu-Natal, South Africa
3. TCCMMP, Theoretical and Computational Condensed Matter and Materials Physics Group, School of Chemistry and Physics, University of KwaZulu-Natal 3 , Pietermaritzburg, South Africa