Molecular dynamics simulations were performed to examine the relative strength of interactions of fluoride 
versus iodide with the ammonium (NH4+) ion, the alkyl ammonium side chain of lysine (RLysNH3+), and the 
zwitterionic ammonium group of glycine  (R(-)GlyNH3+) in aqueous solution.  Clear trends were observed in 
the ammonium- anion association, with iodide showing essentially constant affinity for all three groups, 
whereas fluoride interacted most favorably with the ammonium ion, less with RLysNH3+, and comparably with 
iodide with R(-)GlyNH3+.  Neutron scattering experiments show little difference in the interaction of 
fluoride and iodide with glycine confirming this last observation. The experimental neutron scattering 
data also suggests that the calculated coordination number of fluoride (about 6 in the constant pressure 
simulations) is about 20% too high, indicating certain inaccuracies in the classical force field description 
of this small anion.