1. Contribution from the Departamento de Química Inorgánica, Analítica y Química-Física and INQUIMAE-CONICET, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pab. 2, C1428EHA Buenos Aires, Argentina, Quantum Theory Project and Department of Chemistry, University of Florida, Gainesville, Florida 32611-8435, and Department of Biophysics and Biophysical Chemistry, Johns Hopkins School of Medicine, Johns Hopkins University, Baltimore, Maryland 21205