As a consequence hereof, only C, O and polar hydrogen atoms were used in the fitting procedure. The fitting procedure used two passes to obtain fitted charges. In the first pass, partial charges on all atoms (C, O and polar hydrogen) were allowed to minimize the error with respect to the ESP and in the second pass only charges on C atoms are optimized. The restraint weights were 5.0 • 10 -4 and 1.0 • 10 -4 for the first and second pass, respectively.