Ab initio calculations of cation?neutral diamine complexes have been carried out at the MP2/6-311+G** level. The geometry and energetics of the charge-reinforced hydrogen bond are analyzed with respect to the alkyl substitution of both the protonated and neutral nitrogen atoms, and these results have been used to improve the quality of the MM3(2000) force field. In addition, specialized hydrogen bond parameters optimized for MM3(2000) are presented. These parameters allow very accurate gas-phase modeling of the charge?neutral diamine environment. Molecular mechanics calculations can model effectively protonated amine?neutral amine hydrogen bonds in the gas phase and solution (continuum dielectric) through a combination of charge?dipole interactions and explicit hydrogen-bonding terms.