1. Department of Physics, Molecular Physics Division, Stockholm University, SCFAB, SE-106 91 Stockholm, Sweden, Department of Biotechnology, Theoretical Chemistry Division, Royal Institute of Technology, SCFAB, S-106 91 Stockholm, Sweden, and Department of Clinical Neurosciences, Section for Clinical Neurophysiology, Karolinska Pharmacy and Hospital, Karolinska Institute SE-171 76, Stockholm, Sweden