[1] Univ Vienna, Fac Phys, A-1090 Vienna, Austria
[2] Univ Vienna, Ctr Computat Mat Sci CMS, A-1090 Vienna, Austria
[3] Vienna Univ Technol, Inst Theoret Phys, A-1040 Vienna, Austria
[4] Vienna Univ Technol, Ctr Computat Mat Sci CMS, A-1040 Vienna, Austria